In a large non-stick skillet, heat one tablespoon of butter or oil over medium heat. Add the sliced mushrooms and cook for about 5-7 minutes until they are browned and their moisture has evaporated. Stir in the chopped spinach and cook for another 1-2 minutes until wilted. Season with a pinch of salt and pepper, then remove the vegetables from the pan and set them aside.
In a medium bowl, whisk the eggs with the salt, pepper, and garlic powder until well combined. Using the same skillet, add a little more oil if needed and pour in the eggs. Cook over medium-low heat, stirring gently, until the eggs are softly scrambled and just set. Be careful not to overcook them, as they will cook more inside the quesadilla. Remove the eggs from the pan.
Clean the skillet or use a fresh one. Lay one tortilla flat and sprinkle a generous layer of cheese over one half. Top the cheese with a portion of the scrambled eggs and the mushroom-spinach mixture. Add another sprinkle of cheese on top of the fillings to act as “glue.” Fold the tortilla in half to create a half-moon shape.
Place the folded quesadillas in the skillet over medium heat. Cook for 2-3 minutes per side, or until the tortilla is golden brown and the cheese is completely melted. Repeat with the remaining tortillas. Once finished, let them rest for a minute before slicing into wedges.