Gather all the ingredients. Bring a large pot of water to boil.
Prepare a large jar or container that can fit your tomatoes. Combine all the ingredients for the marinade (½ cup dashi, 1 cup rice vinegar, ¼ cup granulated sugar, 1 Tbsp soy sauce, and 2 tsp kosher salt).
Using a knife, score an X on the tomato skin and place gently into the boiling water.
Blanch for 10-15 seconds, flip the tomatoes once halfway through. Transfer the tomatoes from the pot to the cutting board.
You can dunk them in cold water for 10 seconds or let it cool enough to touch. Gently peel the skin using fingers. Discard the skin.
Place the tomatoes in the marinades and put in the refrigerator for at least 3 hours until completely chilled or keep for up to 3-5 days.
If you like to serve the tomatoes with onion, mince ½ onion. You will need 2-3 Tbsp minced onion per tomato. Use whatever left for other cooking.
Put the minced onion in a bowl and soak in water to remove the bitterness. Squeeze water out and ready to serve on top of the tomatoes.
Cut the tomatoes into ½ inch slices and garnish with minced onion and parsley (if you have any).