Roast the veggies. 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.
½ head cauliflower, ½ small green bell pepper, ½ small red bell pepper, ½ medium yellow onion, 2 tablespoons oil, 2 teaspoons fajita seasoning
Cook the quesadillas. 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.
Make ahead: You could also prep the whole quesadilla, wrap it in plastic wrap, and then cook it in the skillet at a later time. It should last in the fridge for 3 days.To freeze: Cook the quesadilla, then cool before freezing. Wrap well in foil and store in a freezer bag. To reheat from frozen, microwave the frozen quesadilla on HIGH for 2 minutes or toast in a toaster oven for 4-5 minutes.