Preheat oven to 425 degrees F.
Combine tomatillos, 1 cup onion, cilantro, garlic, chile, lime juice and cumin in a blender; puree on high until smooth, about 1 minute.
Spread 1/2 cup of the sauce in a 9-by-13-inch baking dish. Layer 4 tortillas over the sauce. Top with 1 cup potatoes, 1/2 cup cheese and 1/4 cup of the remaining onion. Repeat the layers twice. Top with the remaining tortillas, potatoes and onion. Spread the remaining sauce on top.
Coat a large piece of foil with cooking spray and cover the baking dish tightly. Bake the enchiladas for 10 minutes. Remove the foil and sprinkle on the remaining 1/2 cup cheese. Continue baking until the cheese is melted and the edges are starting to brown, 15 to 20 minutes longer.
To make ahead: Refrigerate sauce (Step 2) for up to 3 days or freeze for up to 3 months.
2 vegetable, 2 starch, 1 high-fat protein