For the filling: In a large skillet, heat oil. Once hot, add the steak and cooked until browned, about 7 minutes. Remove from pan with a slotted spoon and place in a medium bowl. Set aside.
In the same pan, cook onion and bell pepper until tender, 5-7 minutes. Add in garlic and cook until fragrant, about 1 minute. Transfer into the bowl with the steak. Mix to combine. Season to taste with salt and pepper.
For the assembly: In each of 5 tortillas, spread 4 ounces of white queso on half the tortilla. Top with 4 ounces of steak filling and fold tortilla over, forming a half moon.
Cook quesadillas on a flattop or in a skillet over medium-high heat, adding oil as necessary. Quesadillas are done when the outside is golden brown and the insides are hot.
