Place a rack in center of oven; preheat to 425°.

In a large skillet over medium heat, heat 2 tablespoons oil. Add onion, season with 1/2 teaspoon salt, and cook, stirring occasionally, until softened and sweet to the taste, about 7 minutes.

Add taco seasoning and 1 tablespoon oil and cook, stirring, until fragrant, about 30 seconds.

Add beans, remaining 1/2 teaspoon salt, and 1/2 cup water. Bring to a simmer over high heat; reduce heat to medium-low and simmer, mashing down on some beans, until thickened and reduced, 2 to 3 minutes. Remove from heat.

Grease a large metal baking sheet with remaining 2 tablespoons oil. Arrange tortillas in a single layer, making sure that one side of tortilla is coated in oil.

Spoon a scant 1/4 cup bean mixture onto one half of each tortilla. Top each with 2 tablespoons cheese. Fold tortillas over cheese to create a taco. Flip tacos so side with cheese is closest to baking sheet.

Bake tacos until golden brown and crisp on the bottom, 8 to 10 minutes. Flip tacos and continue to bake until golden brown and crisp on the other side, 5 to 8 minutes longer.

Transfer tacos to a platter. Serve with sour cream, cilantro, and lime wedges alongside (if using).