For the Sautéed Spinach and Mushrooms: 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 (this prevents mushrooms from turning black). Saute 3-5 minutes or until mushrooms are cooked through. Then 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.