Heat the olive oil in a pan over medium heat and sauté the sliced mushrooms until they are tender, about 5 minutes.
Season the mushrooms with salt and pepper to taste.
Place one tortilla on a flat surface and sprinkle half of the shredded cheese evenly over it.
Spread the sautéed mushrooms over the cheese and top with the remaining cheese.
Place the second tortilla on top of the filling.
Heat a non-stick skillet over medium heat. Carefully transfer the quesadilla to the skillet.
Cook the quesadilla for about 3-4 minutes on each side or until the tortillas are golden and the cheese is melted.
Remove the quesadilla from the skillet, let it cool for a minute, then cut into wedges.
Serve with sour cream and salsa if desired.