Preheat oven to 375 degrees. Grease a 9-by-13-inch baking dish with oil or non-stick spray.
In a large liquid measuring cup or bowl, mix enchilada sauce, crushed tomato, 1/2 tsp salt, and 1/2 tsp garlic powder.
Add beans to a second bowl and roughly mash about half of the beans with a fork or potato masher. Stir in canned sweet potato puree, spinach, salsa, 1/2 tsp salt, and 1 tsp garlic powder.
To assemble casserole, spread 1 1/2 cups of the sauce mixture in the bottom of the baking dish. Top with a layer of 8 tortilla halves, so that the entire dish is covered. Spread 1/2 of the bean mixture over the tortillas, then sprinkle with 1/3 of the cheese. Repeat one more time with another layer of tortillas, 1 cup sauce, the remaining beans, and a third of the cheese. Top with a final layer of tortillas, then the rest of the sauce and remaining cheese.
Cover with greased or non-stick foil (so the cheese doesn't stick) and bake for 20 minutes. Remove foil, then bake for an additional 10 minutes so cheese is well melted. Allow to cool for 10 to 20 minutes before serving. Top with sliced avocado, a dollop of Greek yogurt, and some fresh cilantro, if desired.