In a medium bowl, combine the black beans, sour cream and roasted vegetables. Season the filling with the salt and black pepper to taste.
Pour 1/4 cup enchilada sauce into the prepared baking dish and tilt it from side to side until the bottom of the pan is evenly coated. To assemble the first enchilada, spread 1/2 cup vegetable filling mixture down the middle of the tortilla, then snugly wrap the left side over and then the right, to make a wrap. Place it seam side down against the edge of your pan. Repeat with the remaining tortillas and filling.
Drizzle the remaining sauce evenly over the enchiladas leaving the tips of the enchiladas bare. Sprinkle the cheese evenly over the enchiladas, followed by the pepitas, if using.
Bake, uncovered, on the middle rack for 20 minutes. If the cheese on top isn't golden enough for your liking, carefully transfer the enchiladas to the upper rack of the oven and bake for an additional 3 to 6 minutes, until sufficiently golden and bubbling.
Remove from the oven and let the enchiladas cool for 10 minutes (they're super hot). Before serving, arrange thin slices of avocado down the center of the enchiladas and sprinkle with chopped cilantro. Serve with sour cream on the side, if you'd like. Leftovers store well, chilled, for up to 4 days. Gently reheat in the microwave or oven before serving.