Preheat the oven to 400 degrees. Prepare a 9 x 13-inch baking dish with nonstick cooking spray.

Heat a large heavy skillet over medium to medium-high heat; add the oil and cook zucchini and onions for 2 minutes. Turn heat down and cook until tender, about 8 minutes. Remove from the pan into a bowl.

In the same pan, cook the ground beef; season to taste and drain off the fat. Add the beans and veggie mixture to the drained beef; gently stir and cook on low heat for 2 minutes.

Combine the salsa, water, and pumpkin in a blender; puree until smooth and creamy. (Add more water if too thick, depending on what type of salsa you use.) Add the cumin and hot sauce; mix well. Taste with your finger; adjust seasoning as needed.

Pour 1 cup of the sauce into the bottom of the baking dish; spread evenly with a spatula. Top with 6 overlapping tortillas. Next add 1/2 the meat and veggie mixture, 1/2 the remaining sauce, and ½ the cheese. Repeat the layers with the remaining tortillas, vegetable beef mixture, and sauce, ending with a layer of cheese.

(At this point you can cover and refrigerate the casserole for up to 24 hours, and bake later if desired.)

Spray a piece of foil, cover the dish with the sprayed side down, and bake for 25 minutes.

Remove the foil and bake for 5 more minutes. Let the casserole cool for 10 minutes; cut into squares to serve, topped with sour cream.