Heat olive oil on a griddle or saucepan over medium-high heat. Add onions and peppers and cook until onions are caramelized, about 6 minutes.
Add garlic, salt, and pepper, cook and stir for about 30 seconds. Push the veggies to the side of the pan.
Add steak to pan, cooking while breaking up the meat with the back of 2 spatulas, until no longer pink. Once cooked through, mix in the peppers and onions.
Top the steak and veggies with slices of cheese and let melt.
Divide steak and veggies between the two croissants.