Turn the broiler on high, with the oven rack set close to the top.

Halve the tomatoes lengthwise. Add the tomatoes, onion, jalapeños, and garlic cloves (skin still on — it keeps the garlic from burning) to a foil-lined sheet tray, cut sides down.

Roast under the broiler for about 10 minutes, or until the tomato skins are charred and look papery.

Remove from the oven. Peel the roasted garlic cloves out of their skins, then add everything (including the peeled garlic) to a blender, along with the cilantro, salt, lime juice, and cumin.

Pulse a few times until combined but still a little chunky — you don't want a purée.

Heat about 1 tablespoon of neutral oil (vegetable oil works great) in a skillet over medium-high heat until hot.

Pour the salsa into the skillet and flash-fry, stirring constantly, for 2 to 3 minutes. This cooks off a little excess water and concentrates the flavor.

Pour into a container to cool, then store in the fridge.