Slice the bell peppers, zucchini, and red onion into thin strips or small dice for even cooking.
Heat a skillet over medium heat and add olive oil, allowing it to shimmer slightly.
Add the chopped red onion to the skillet and sauté until it becomes translucent and fragrant, about 2-3 minutes.
Stir in the bell peppers and zucchini, cooking for another 3-4 minutes until the vegetables are tender and slightly caramelized around the edges.
Season the vegetable mixture with salt and pepper, then remove from heat and set aside to cool slightly.
Lay a flour tortilla flat on a clean surface and evenly sprinkle half of the shredded cheese over one half of the tortilla.
Spread a portion of the sautéed vegetables over the cheese layer, then sprinkle with the remaining cheese.
Fold the tortilla in half to enclose the filling, pressing down gently to flatten slightly and seal the edges.
Place the assembled quesadilla onto the preheated skillet and cook for about 3-4 minutes on each side until the tortilla is golden brown and crispy, and the cheese has melted inside.
Use a spatula to flip the quesadilla carefully, ensuring even browning and that it doesn’t fall apart.
Once both sides are crispy and the cheese is bubbly, remove the quesadilla from the skillet and let it rest for a minute before slicing.
Cut the quesadilla into wedges and serve warm, garnished with your favorite toppings like cilantro or a squeeze of lime.
