Preheat oven to 350°F and grease a rimmed baking sheet with oil.
Sauté onions in a skillet until soft. Add ground beef and cook until browned. Stir in taco seasoning, salt, and black pepper. Drain excess fat.
Arrange 10 tortillas overlapping on the sheet pan, letting the edges hang over.
Layer beef, cheese, and pico de gallo evenly over the tortillas.
Place 6 tortillas on top and fold over the overhanging edges to enclose the filling. Brush the top with oil.
Cover with a second sheet pan and press down with a heavy skillet. Bake for 30 minutes, then remove the weight and bake another 30 minutes until golden.
Let cool for 5 minutes, then slice into portions and serve with toppings.