Cook the steak: Season the sliced steak with salt and pepper (or taco seasoning if desired). Heat 1 tsp olive oil in a skillet over medium-high heat. Add steak and cook for 3–5 minutes, stirring occasionally, until browned and cooked through. Remove from skillet and set aside.
Sauté the veggies: In the same skillet, add a little more oil if needed. Sauté the sliced bell pepper for 3–4 minutes until softened. Add diced tomato and cook for 1 more minute. Season lightly with salt and pepper. Remove from skillet.
Assemble the quesadillas: Lay tortillas flat. On half of each tortilla, layer shredded cheese, cooked steak, sautéed peppers and tomatoes, then more cheese. Fold the other half over to close.
Cook the quesadillas: Wipe out the skillet if needed. Add a bit of butter or oil and heat over medium. Cook each quesadilla for 2–3 minutes per side, until golden brown and the cheese is melted.
Serve: Let cool slightly, then cut into wedges. Serve with sour cream, salsa, or guacamole if desired.
