Preheat oven to 375°F (190°C). Pour about 1/2 cup of enchilada sauce into the bottom of a 9x13 inch baking dish and spread to coat.

In a large skillet, heat olive oil over medium heat. Add the chopped onion and cook until softened, about 5-7 minutes. Add the minced garlic and cook for another minute until fragrant.

Stir in the chili powder, cumin, and oregano. Cook for 30 seconds until fragrant. Add the rinsed black beans and 1/4 cup of the enchilada sauce. Cook, stirring occasionally, and lightly mash about half of the beans with a spoon to create a creamy texture. Remove from heat.

In a separate small skillet, heat the vegetable oil over medium-high heat. Using tongs, fry each tortilla for 10-15 seconds per side until softened and pliable. Place on a paper towel-lined plate.

To assemble, spoon about 3 tablespoons of the black bean mixture down the center of a tortilla. Top with a sprinkle of shredded cheese. Roll tightly and place seam-side down in the prepared baking dish. Repeat for all tortillas.

Pour the remaining enchilada sauce evenly over the top of the enchiladas. Sprinkle with the remaining cheese.

Bake for 20-25 minutes, until the sauce is bubbly and the cheese is fully melted and slightly golden. Let cool for 5-10 minutes before serving.

Garnish with fresh cilantro, sour cream, and avocado if desired.