Heat the oil in a large frying-pan over a medium heat. Add the onions and mushrooms and cook until the mushroom water is evaporated and they begin to brown, 5 to 7 minutes. Add the garlic and cook for one minute more. Add chicken, cumin, chili powder and oregano and stir until all spices are incorporated. Add spinach, salt and pepper and cook until spinach is wilted, about two minutes.
Lay one tortilla on a flat work surface and sprinkle with 20g cheddar cheese. Spoon half the chicken and vegetable mixture on top of cheese, then top with an additional 20g cheese. Top with another flour tortilla. Heat a large nonstick frying-pan with cooking spray over medium heat. Carefully place 1 quesadilla in pan and cook for three minutes. Using a large spatula, gently flip quesadilla and cook for an additional three minutes until lightly browned and cheese is melted. Repeat with second quesadilla. Slice each quesadilla into quarters. Place two quarters on a plate with one tablespoon sour cream and two tablespoons salsa.