In a large pan or skillet, heat olive oil over medium heat. Once the oil is heated and shimmering, add steak and stir occasionally until browned, about 3-4 minutes. Remove steak from the pan and set aside.
Add onion and bell pepper to the pan. Stirring occasionally, cook until softened but the pepper still has a slight crunch, about 4-6 minutes.
Add the steak back into the pan, pour in the taco seasoning, and stir until everything is evenly combined.
Set aside the steak mixture and wipe all excess oil out of the pan. Turn the heat down to medium-low.
Assemble the quesadilla. On half of the tortilla, sprinkle an even layer of 1⁄4 cup of cheese. Top with the steak and vegetable mixture, top with another 1⁄4 cup of cheese, and fold the tortilla in half.
Place quesadilla in the pan (dry, no added oil). Let cook on one side for 2-3 minutes, or until cheese begins to melt and the tortilla on the bottom is golden brown. Turn down the heat if the tortilla is cooking too fast and the cheese is not melting.
Flip and cook on the other side for another 2-3 minutes, or until the tortilla is golden brown.
Remove quesadilla from heat and repeat with remaining ingredients. Serve immediately with sour cream, pico de gallo, and cilantro.