Preheat the oven to 400F with racks in the middle and upper third of the oven. Line a large rimmed baking sheet with parchment for easy cleanup. Lightly grease a 9x13-inch baking dish.

On the prepared baking sheet, combine the sweet potato and poblanos. Drizzle the olive oil over them and toss until all sides are lightly and evenly coated, adding more oil if necessary. Lightly sprinkle with salt and pepper and arrange the ingredients in a single layer. Bake on the middle rack until the sweet potato and poblanos are tender and caramelized on the edges, 40 to 45 minutes, tossing halfway. Leave the oven on for the enchiladas.

In a medium saucepan, warm the oil over medium heat until it's so hot that a sprinkle of the flour and spice mixture sizzle on contact. Pour in the flour and spice mixture. While stirring constantly with a whisk, cook until fragrant and slightly deepened in color, about 1 minute. Whisk the tomato paste into the mixture, then slowly pour in the broth while whisking constantly to remove any lumps.

Increase the heat to medium high and bring the mixture to a simmer, then reduce the heat as necessary to maintain a gentle simmer. Cook, whisking often, until the sauce has thickened a bit and a spoon encounters some resistance as you stir, 5 to 7 minutes. Remove from the heat, then whisk in the vinegar. Season to taste with a generous amount of black pepper and extra salt, if necessary. Set aside.

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 ¼ 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 ½ 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. 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.