Season the Steak: In a bowl, mix the steak slices with garlic powder, chili powder, cumin, salt, and black pepper. Let it marinate for 10 minutes.
Cook the Steak: Heat olive oil in a pan over medium-high heat. Add the steak and cook for 2-3 minutes per side until browned and cooked to your liking. Remove and set aside.
Sauté the Veggies: In the same pan, add the onion and bell pepper. Cook for 3-4 minutes until soft and slightly caramelized. Remove from heat.
Assemble the Quesadilla: Heat butter in a clean pan over medium heat. Place one tortilla in the pan, sprinkle cheese, add steak and veggies, then more cheese. Top with another tortilla.
Cook & Flip: Cook for 2-3 minutes until golden brown, then carefully flip and cook for another 2-3 minutes until the cheese melts and the tortilla is crispy.
Slice & Serve: Remove from the pan, let it rest for a minute, then cut into wedges. Serve hot with sour cream, guacamole, or salsa.