Preheat the oven to 375°F.
Heat the oil in a large skillet over medium heat. Add the scallion whites and cook, stirring, until just tender, about 30 seconds. Add the beans, taco seasoning, and 1 cup water. Cook, smashing the beans, until half are mashed and the mixture is thick, about 5 minutes.
Divide the beans among the tortillas, spreading them in a line down the center of each. Sprinkle the bean mixture with half of the cheese. Roll up each tortilla and arrange snugly in a 3-quart shallow baking dish, seam side down. Pour the taco sauce on top and sprinkle with the remaining cheese.
Cover with foil and bake until hot and bubbly, about 20 minutes.
Top with the avocado, peppers, and scallion greens and serve hot.