Preheat oven to 200F.

Wash tomatoes, cut off the stem end, and halve lengthwise. Pour a bit of olive oil into a small bowl, dip a pastry brush into it, and brush the tomato halves lightly with oil. Place them, skin side down, on a large baking sheet. Sprinkle them with sea salt and ground coriander—about a pinch of each for every four to six tomato halves.

Bake the tomatoes until they shrink to about 1/3 of their original size but are still soft and juicy, 4 to 6 hours. Remove the baking sheet from the oven, and allow the tomatoes to cool to room temperature. Place them in an airtight container, and store them in the refrigerator.