In a skillet over medium heat, melt butter. Add garlic and mushrooms, season with sea salt and squeeze in the juice of 1 lemon wedge. Sauté 3-5 minutes or until mushrooms are cooked through. Add spinach and mix. Cook just until spinach is wilted, remove from heat.
Assemble your quesadilla by spreading pesto on one side of a tortilla, then top with sautéed spinach/mushrooms and cheese. Place another tortilla on top to make a quesadilla.
Fry your quesadilla in butter for 1-2 minutes per side over medium heat or until lightly brown and cheese is melted.
Transfer to a plate and cut into pieces, serve with pizza sauce, if desired.