Preheat your oven to 350°F (175°C).
In a large skillet, heat olive oil over medium heat.
Add sliced onions and bell peppers; sauté until softened.
Add the thinly sliced flank steak to the skillet.
Season with garlic powder, salt, and black pepper.
Cook until the steak is browned, about 5-7 minutes.
Layer half of the steak mixture in a greased casserole dish.
Sprinkle 1 cup of provolone cheese over the steak mixture.
Add the remaining steak mixture on top, followed by the remaining provolone and cheddar cheese.
Bake in the preheated oven for 20-25 minutes until the cheese is bubbly and golden.