Heat a skillet over high heat.
Add steak and sear for 30 seconds per side, then cook 2½ minutes more, flipping once.
Add butter, thyme, and garlic. Baste steak for 1 minute.
Rest for 5 minutes, then slice thinly against the grain.
In the same skillet, add oil if needed.
Cook onions, bell peppers, and corn with 2 tsp of BBQ seasoning until softened and lightly charred (about 3 minutes). Season with salt and pepper.
Heat a clean skillet over medium.
Lay 1 tortilla flat, sprinkle half the cheese on one side.
Top with steak slices, sautéed veggies, and remaining cheese.
Fold the tortilla in half and press gently.
Cook the folded quesadilla for 2–3 minutes per side, until golden brown and the cheese is melted. Repeat with the second quesadilla.
Cut into wedges. Serve with lime wedges, sour cream, salsa, or hot sauce!