Note: if your blender has a small jar, make this salsa in two batches. Makes about 2 cups.
1. Roast tomatillos on a rimmed baking sheet about 4 inches below a very hot broiler until soft and blackened in spots, about 5 minutes per side. Cool and then scrape tomatillos and juices into blender jar.
2. Heat a small ungreased skillet on medium. Add dried red chipotle chiles. Stirring constantly, toast the chiles until very aromatic, about one minute. Scoop into a small bowl, cover with hot water and soak for about 30 minutes, stirring occasionally for even rehydration.
3. Turn the oven down to 425 F. Spread the onion slices and whole peeled garlic cloves on another baking sheet. Roast for about 15 minutes. Stir every few minutes until the onions are golden and look wilted. The garlic should be soft and browned in spots. Cool.
4. To finish the salsa, drain the chiles and scrape them into blender jar with the tomatillos. Add 3/4-cup cold water, the roasted garlic and salt. Blend until nearly smooth. Roughly chop the roasted onion and add to the blender. Pulse a few times or until the salsa is as chunky or smooth as you like it. Stir in more water, if needed, if salsa is too thick.
5. Taste and add more salt, if needed. Serve cold, hot or at room temperature or use in recipes that call for salsa verde. Makes about 2 cups.