Preheat oven to 400°F (200°C) and arrange halved Roma tomatoes, jalapeño peppers, quartered onion, and unpeeled garlic cloves on a baking sheet.
Roast vegetables for 20–25 minutes, until tomatoes are softened and slightly charred, jalapeños and onion are tender, and garlic skins are browned.
Allow roasted vegetables to cool until manageable, then peel skins from the garlic cloves.
Place roasted tomatoes, jalapeños, onion, peeled garlic, chopped cilantro, salt, and 1/4 cup water into a blender.
Blend to desired consistency, adding water by tablespoons if too thick.
Pour salsa into a saucepan, bring to a gentle simmer over medium–low heat, and cook for about 10 minutes, stirring occasionally.
Taste and adjust salt, then serve warm or chilled with chips, tacos, or other dishes.