Preheat the oven to 425ºF. Set a wire rack into a rimmed baking sheet and then spray with nonstick cooking spray. Set aside.
Prepare the chicken first, by mixing the shredded chicken and salsa together in a bowl and toss. Set aside so the chicken absorbs as much salsa as possible. The type of salsa you use dictates how spicy your quesadilla is.
Heat a large skillet over medium/high heat. Add olive oil. When the olive oil is fragrant, add the onion to the pan and sprinkle with salt. Sauté the onions for 3-4 minutes.
Add the garlic and tomato to the onion and sprinkle with taco seasoning. Cook the tomatoes down for 6-8 minutes or until the tomatoes have bursted and started to soften.
Next, add the bell pepper and spinach to the pan and toss them into the other vegetables. Cook for an additional 3-4 minutes or until the spinach is wilted and there isn’t a ton of excess liquid in the pan. Remove from heat and set aside.
Line the wire rack with tortillas. Add 2 tortillas flat in the middle of the rack. Then, line the longer outside edges with 3 tortillas, they should be overlapping. Add 2 more tortillas to the two shorter edges so that the cooling rack is completely covered.
Spread the shredded chicken over the middle of the tortillas, but not completely to the edges of the tortillas, you will be folding the edges over the quesadilla filling.
Spread the cooked vegetables evenly over the chicken and then sprinkle the shredded cheese over the vegetables.
Place the last 2 flour tortillas in the middle of the quesadilla filling and then carefully fold the edges of the tortillas up over the filling. Be sure the tortillas are overlapping so that the filling is secure.
Place an additional baking sheet on top of the sheet pan quesadillas. This is so the edges don’t curl. Bake the quesadillas at 425ºF for 20 minutes.
Remove the top baking sheet and drizzle with olive oil.
Bake for an additional 8-10 minutes or until the quesadilla is golden brown.
Remove the quesadilla from the oven and let sit for 5 minutes before transferring to a large cutting board.
Use a large knife or pizza cutter to cut the quesadillas into 9 squares.
Top with guacamole, sour cream, and fresh cilantro.