Heat olive oil in a skillet over medium heat. Add chopped onion and cook until translucent.
Add sliced steak to the skillet and cook until browned.
In a baking dish, layer corn tortillas, steak mixture, enchilada sauce, and shredded cheese. Repeat layers until all ingredients are used, ending with cheese on top.
Bake in the preheated oven for 30-40 minutes, or until cheese is bubbly and golden.
Let the casserole cool for a few minutes before serving.