The description you gave is very similar to a classic Korean side dish, often simply called 토마토 장아찌 (Tomato Jangajji)—a soy sauce-pickled tomato.
The claim that they “keep until New Year’s without canning” is absolutely true if made correctly, thanks to the preservative nature of salt, sugar, and the fermentation or high-acid environment.
Here is the most likely recipe based on your description, focusing on that incomparable savory-sweet-tangy taste and long storage.
Korean Soy-Pickled Tomatoes (토마토 장아찌)
This isn’t a fermented kimchi in the bubbly sense; it’s a quick-pickled banchan that matures and deepens in flavor over time in the fridge.
Ingredients
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Tomatoes: 1 kg (about 5-6 medium), firm and not overripe. Small to medium cocktail tomatoes or firm Roma tomatoes work best. Cherry tomatoes are excellent.
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For Blanching & Peeling:
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Boiling water
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Ice water
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For the Pickling Liquid (The “Jang” base):
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1 cup (240ml) Soy sauce (use a good quality, naturally brewed one like Sempio or Kikkoman for a cleaner taste)
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1/2 cup (120ml) Rice wine vinegar (or apple cider vinegar, or a mix with white vinegar)
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1/2 cup (120ml) Water
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1/3 cup (70g) Sugar (adjust to your taste; you can use honey or oligosaccharide/조청 for a more complex Korean sweetness)
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1/4 cup (60ml) Korean plum extract (매실청 maesil-cheong – this is a key flavor booster, add extra sugar if you omit it)
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1 tsp Salt
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Aromatics (A flavor pouch is best to keep the liquid clear):
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4-5 cloves Garlic, thinly sliced
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1 small knob Ginger, thinly sliced
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1/2 an Onion, thinly sliced
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1-2 Red chili peppers (청양고추 or jalapeño), sliced (optional, for a slight kick)
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A small piece of dried kelp (다시마 dashima), about 2×2 inches
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Instructions
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Prepare the Tomatoes: Gently score a shallow “X” on the bottom of each tomato. Blanch in boiling water for 20-30 seconds, then immediately shock in ice water. The skins will slip right off. Peeling is essential for the sauce to really permeate the tomato flesh and create that silky texture.
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Make the Pickling Brine: In a saucepan, combine the soy sauce, vinegar, water, sugar, plum extract, and salt. Bring to a boil over medium heat, stirring to dissolve the sugar. Once boiling, add the garlic, ginger, onion, chilies, and kelp. Immediately turn off the heat and let the brine cool down completely. The residual heat will draw out the aromatics’ flavors. Remove the kelp after 10 minutes, or it will make the brine slimy.
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Pack the Jar: Use a clean, sterilized glass jar. Gently place the peeled, whole tomatoes inside. If they are large, you can halve or quarter them, but whole cherry tomatoes are magical.
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Pour the Cooled Brine: Once the brine is completely cool to room temperature, strain it into a bowl to remove the boiled aromatics (they’ve done their job and will cloud the liquid and spoil faster). Pour the clear, flavorful brine over the tomatoes, ensuring they are fully submerged. A small fermentation weight or a crumpled piece of clean parchment paper on top helps keep them under the liquid.
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The Waiting Game (Crucial for Taste):
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Room Temperature: Let the jar sit at a cool room temperature, out of direct sunlight, for 1 day (24 hours) . This jump-starts the flavor penetration.
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Refrigerator: Transfer the jar to the refrigerator. The tomatoes will reach their “incomparable” peak flavor after 3 days, and the taste will continue to mature and deepen for weeks.
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Why This Keeps Until New Year’s
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Cleanliness: Using a sterilized jar and clean utensils is step one.
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Acid & Salt: The combination of vinegar and soy sauce creates a high-acid, high-salt environment that is very inhospitable to mold and bad bacteria.
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Sugar: Sugar is also a natural preservative at high concentrations.
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Refrigeration: Stored at a consistently cold fridge temperature (below 4°C / 39°F), the maturation process slows to a crawl, preserving the tomatoes in a delicious, suspended state.
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Submersion: As long as the tomatoes are always below the surface of the liquid, they are sealed off from airborne mold spores. Always use a clean spoon to take them out.
To serve, slice a chilled, soy-pickled tomato, drizzle with a tiny bit of sesame oil, and sprinkle with toasted sesame seeds and sliced green onion. It’s an unbelievably refreshing, complex, and savory banchan that cuts the richness of any meal. The leftover liquid is liquid gold and makes an incredible salad dressing base.