Heat the oven to 375°F and lightly oil a 9-by-13-inch baking dish.
Brown the beef and onion in a skillet over medium-high heat, drain well and stir in the taco seasoning and beans.
Brush the tortillas lightly with oil, stack them in foil and warm in the oven for 8 minutes, then cut them in half.
Spread 1/2 cup sauce in the dish and build three layers of tortillas, beef mixture, sauce and cheddar, ending with cheese.
Cover and bake for 22 minutes, then uncover and bake 10 to 13 minutes until bubbling and browned at the edges.
Rest for 15 minutes before cutting into squares.