Set an oven rack about 6 inches from the broiler and preheat the broiler. Arrange the tomatoes cut side up on a foil-lined sheet pan and scatter the chiles, onion, and garlic around them. Broil for 8 to 10 minutes, until the tomato skins are blackened in spots and the chiles have darkened.

Let the vegetables cool briefly, then peel the garlic. Transfer everything to a blender with the cilantro, salt, and water. Blend for 30 seconds until almost smooth, scraping down the sides as needed.

Pour the salsa into a bowl, top with extra cilantro and chopped onion, and serve. It’s delicious warm, at room temperature, or chilled.