Pour in the sauce and cubed squash and stir well (watch out for splattering as the sauce hits the hot oil).
Salt to taste (try about 1 teaspoon to start).
Reduce the heat to medium low and cook until the squash is tender.
Garnish with queso anejo (aged white cheese) or queso fresco (fresh white cheese), cilantro, and toasted pumpkin seeds (pepitas).
Adapted from Rick Bayless's Mexican Kitchen
Tomatillos can be found in many grocery stores and nearly all Latin markets. Unlike tomatoes, they are pretty good in the off-season.
Chiles can be complicated: There are two ways to use chipotle chiles in this recipe, dry or canned. Either way, you end up with a puree of the chile.
For canned chiles, open a can of chipotle chiles in adobo sauce (available at Latin American groceries and even some upscale grocery stores). Pour the whole thing into a blender jar. Blend until smooth. Scoop out the puree onto plastic wrap in 1 tbsp blobs or as a big log. Store in the freezer. When you need some chipotle puree, simply slice off what you need. (This is my preferred method -- it's so easy.)
For dried chiles, remove the stem and seeds (be sure to wash your hands well after touching the chiles), then soak them in hot water for about 30 minutes. Drain and put the chiles in the blender jar. Discard the soaking liquid.