Preheat the oven to 425°F. Place the cut vegetables on a baking sheet and drizzle with the olive oil. Toss to coat. Sprinkle the veggies with the fajita seasoning and toss again. Roast the vegetables for 25-30 minutes, flipping the vegetables halfway through if desired. Remove from the oven and set aside.
Melt the butter in a large non-stick skillet over medium. Add one tortilla and sprinkle with half of the cheese. Top with the roasted vegetables. Sprinkle with the remaining cheese and top with the other tortilla. Cover with a lid and cook for 3-5 minutes or until golden on the bottom. Lower the heat if the tortillas brown too quickly. Flip the quesadilla, cover, and cook for another 3-4 minutes or until the cheese is melted.
Cut into wedges and serve with salsa or pico de gallo.