Pack a quart jar with tomatoes, garlic, peppercorns, and salt. Add enough water to cover, leaving about 1" head space. Place the lid on the jar and shake to dissolve salt.
Place a fermenting weight on top of the tomatoes in the jar, cover jar, and let rest at room temperature for 4-7 days.
After a few days sample the tomatoes. Once they are fermented to your liking, move to the refrigerator.
Made with small, immature San Marzano tomatoes slit in half lengthwise. Gently crushed the garlic before adding, and added about 4 cloves along with one whole, dried chile de arbol.