Step 1: Roast the Butternut Squash : Preheat oven to 400°F (200°C). Toss the cubed squash with olive oil, salt, and pepper. Spread on a baking sheet and roast for 20–25 minutes, until tender and slightly caramelized.
Step 2: Prepare the Filling : In a large skillet, sauté the chopped onion in a little oil over medium heat until translucent (about 3 minutes). Add garlic, cumin, chili powder, and smoked paprika; cook for 30 seconds until fragrant. Stir in roasted butternut squash and black beans. Season with salt and pepper. Add chopped cilantro if using. Remove from heat and let the filling cool slightly.
Step 3: Make the Creamy Sauce : In a medium saucepan, melt butter over medium heat. Whisk in flour and cook for 1 minute to make a roux. Slowly whisk in broth, stirring until smooth and thickened (about 3–4 minutes). Stir in sour cream, salsa verde, cumin, salt, and pepper. Heat gently, but don’t boil, to keep the sauce creamy.
Step 4: Assemble the Enchiladas : Grease a 9x13-inch baking dish. Spread a thin layer of sauce on the bottom. Spoon some filling into each tortilla, roll it up, and place seam side down in the dish. Pour remaining sauce evenly over the rolled tortillas. Sprinkle shredded cheese on top.
Step 5: Bake : Lower oven temperature to 375°F (190°C). Bake uncovered for 20–25 minutes, or until the cheese is melted and bubbly. For a golden top, broil for 2 minutes at the end (watch carefully).