In a large skillet over medium heat, add 1 tablespoon oil. Add the sliced onions and cook them until they are lightly caramelized.
Add sliced mushrooms and cook until tender for about 5 minutes on medium heat. Do not cover after adding the mushrooms so any excess moisture can be evaporated.
Add the spinach, a little bit at a time, and mix it until the spinach is wilted. Remove from heat.
Heat a large griddle or pan on medium heat. Place a tortilla on the pan, apply little oil on both sides and cook for a minute, flipping with a spatula until the tortilla is hot. Spread a thin layer of the cooked veggies on the tortilla. Spread a layer of shredded cheese on top.
Carefully place the second tortilla on top of the cheese and press it gently with the spatula. Add few more drops of oil while pressing gently. Pressing will help the quesadilla stick together and should be easier to flip it over. Cook until both sides are nice and crispy golden brown and the cheese starts to melt! Gently put the quesadilla on a cutting board and cut into 4(more or less) slices using a pizza cutter.
Serve with fresh guacamole and salsa.
Cheese - We love pepper jack cheese in this recipe. I have also used mozzarella cheese and it tastes delicious. To make vegan quesadillas, try Miyoko's Creamery Vegan Cheese. Is there a vegan cheese you prefer? Please comment below, I will love to know and it will also benefit other readers.
Tortillas - I usually make these quesadillas with whole wheat tortillas, trader joes sundried tomato, and habanero tortillas or corn tortillas for gluten-free quesadillas. Recently I also tried making these quesadillas with Siete Foods almond flour tortillas that are paleo-friendly and vegan. The taste and texture are spot on. Many local grocery stores including whole foods carry Siete brand tortillas.