Preheat oven to 425º F. Line a baking sheet with parchment paper and set aside.
Onto the baking sheet, add the cubed butternut squash. Drizzle with olive oil, and season with salt and pepper. Bake the butternut squash for 20 minutes.
Add the sliced red pepper to the baking pan in the oven along with the squash, and allow to bake an additional 10 minutes. Remove the vegetables from the oven, but leave the oven on if baking the enchiladas soon.
Warm tortillas in a skillet or wrapped in a towel set in the still warm oven. While the tortillas are warming, prepare other ingredients. Shred the cheddar cheese, open the black beans and drain them, and place the enchilada sauce in a wide shallow bowl.
Assemble the enchiladas by dipping a warmed tortilla in enchilada sauce, and then adding the tortilla to a large pan (9×13 baking dish). Into the center of the enchilada, add a large spoonful of the butternut squash and red pepper mixture. Add 1 tablespoon of black beans, along with a few leaves of spinach, and a sprinkle of cheddar cheese. Roll the enchilada and place the seam-side down in the pan to hold the roll together.
Repeat with all tortillas, lining the enchiladas in a single layer to fill the entire bottom of the baking pan.
Pour remaining enchilada sauce over the tops of the rolled enchiladas. Top with remaining cheese.
Cover the pan with aluminum foil, and bake the enchiladas for 30 minutes. Remove the foil, and bake uncovered for an additional 10-15 minutes, until the sauce is bubbling and the cheese has begun to brown on the top.
Garnish the enchiladas with cilantro if desired. Serve with avocado, sour cream, or hot sauce, if desired.