Preheat broiler. Place tomatoes, onion, jalapenos, and garlic on a rimmed baking sheet. Broil until tomatoes and jalapenos are charred, about 7 minutes. Let stand until cool enough to handle.
Peel the roasted garlic. Transfer to a blender along with the charred tomatoes, onion, jalapenos, and 1 tablespoon salt. Puree until smooth. For a thinner consistency, add water as needed. Season with salt to taste.
This salsa can be covered and refrigerated for up to 2 weeks.