Marinate the Steak. In a bowl, combine the sliced steak, olive oil, minced garlic, chili powder, cumin, salt, and pepper. Allow the steak to marinate for at least 30 minutes.
Cook the Steak. Heat a skillet over medium-high heat. Add the marinated steak slices and cook for 2-3 minutes per side, or until browned and cooked to your desired level of doneness. Remove from heat and set aside.
Assemble the Quesadillas. Lay a tortilla flat on a clean surface. Sprinkle a layer of shredded cheese on one half of the tortilla. Top with cooked steak slices, bell pepper, onion, and another layer of cheese. Fold the tortilla in half to enclose the filling.
Cook the Quesadillas. Heat a skillet or griddle over medium heat. Place the assembled quesadillas in the skillet and cook for 2-3 minutes on each side, or until the tortillas are golden brown and the cheese is melted.
Serve and Enjoy. Remove the quesadillas from the skillet and slice into wedges. Garnish with chopped cilantro and serve with sour cream, salsa, and guacamole on the side.