Step 1

Heat a very large skillet over medium high heat. Add the oil, and when hot add the mushrooms and onions, and season with salt and pepper. Sauté for about 8 minutes until onions have softened and turned slightly golden brown, and the mushrooms are browned and any liquid that was released from the onions have evaporated. Add the spinach and saute for another 1 or 2 minutes until the spinach is wilted. Remove them to a bowl and stir in the chicken until everything is evenly mixed.

Step 2

Wipe out the skillet and return it to medium high heat. Add a half teaspoon of butter, and turn to coat the bottom of the pan. Place a tortilla in the pan. Sprinkle 3 tablespoons of the cheese over half of the quesadilla, and distribute 1/8 of the chicken and vegetable mixture over the cheese. Top that with another 3 tablespoons of the shredded cheese. Flip the bare half of the tortilla over the filling, cover the pan, and sauté for about 2 minutes until the bottom is golden and the cheese has started to melt, then use a spatula to flip the half-moon quesadilla, and continue to cook, uncovered, until all of the cheese is melted and the underside is browned, 2 to 3 minutes. You can cook two at a time, if you nestle them in the pan with the flat sides of the half moons abutting each other.

Step 3

Remove the quesadilla to a cutting board and let the quesadilla sit for a minute before you slice into 2 or 3 wedges. Repeat until all of the quesadillas are cooked. Serve with salsa, guacamole and sour cream, as desired.