Preheat oven to 400°F. Heat oil in a large skillet over high. Add beef; cook, stirring often, until browned, 6 minutes. Add half of onion to skillet. Cook, stirring often, until softened, 5 minutes.

Add tomatoes, beans, taco seasoning, and 1/2 cup water to skillet, stirring to combine. Bring to a boil; reduce heat to medium. Simmer, stirring often, until slightly reduced, 3 minutes. Remove from heat.

Heat tortillas in a dry skillet over high, 15 seconds per side. Transfer to a plate; cover with a paper towel to keep warm.

Place 3 tortillas in an 8-inch square baking dish. Top with 2 cups of the beef mixture; sprinkle with 1/2 cup of the cheese. Repeat layers 2 more times. Bake in preheated oven until cheese is melted, 8 minutes. Remove from oven. Top with lettuce, pico de gallo, and remaining onion.