Preheat the oven to 400 degrees F. Place sweet potatoes on a large baking sheet. Drizzle with 1 tablespoon of the olive oil and toss until sweet potatoes are coated. Season with salt and pepper, to taste. Roast sweet potatoes for 30-35 minutes, or until tender, stirring occasionally. Remove sweet potatoes from oven and set aside. Reduce oven temperature to 350 degrees F.

While the sweet potatoes are roasting, work on the rest of the filling. Heat a large skillet over medium-high heat and add the remaining 1 tablespoon of olive oil. Add the onion and red pepper. Cook until tender, about 3 minutes. Stir in the garlic and spinach leaves and cook until spinach leaves are wilted, about 3 minutes. Add the black beans, cumin, chile powder, lime juice, and cilantro. Season with salt and pepper, to taste. Remove from heat and stir in the roasted sweet potatoes.

Grease an 8x8 square baking pan with nonstick cooking spray. Spread 1/4 cup of enchilada sauce into the bottom of the baking pan. Add a layer of tortillas, to completely cover the bottom of the pan. You might have to cut the tortillas to make them fit. Top with a little more enchilada sauce, 1/3 of the sweet potato bean mixture, and 1/3 of the cheese. Make a second layer of tortilla, enchilada sauce, sweet potato bean mixture, and cheese. Top with with a layer of tortillas, enchilada sauce, sweet potato bean mixture, and cheese. Spray a sheet of aluminum foil with cooking spray and cover the pan.

Bake enchiladas for 20 minutes. Remove the foil and bake for another 10 minutes, or until cheese is melted and the enchiladas are bubbling. Remove enchiladas from the oven and let cool for 10 minutes. Top with diced avocado. Cut into squares and serve warm.