Warm up 2 baking sheets in your oven, setting the temperature to a low heat between 170 and 190°F. Cut 2 pieces of aluminum foil to match the size of your baking sheets, and set them aside. These will be used to keep your quesadillas warm until it's time to serve.
Heat a non-stick skillet over medium high heat.
Add olive oil to the skillet, along with mushrooms, onion, garlic, and a large pinch of salt.
Cook until the liquid from the mushrooms is fully evaporated.
Incorporate the spinach, another pinch of salt and some fresh ground pepper. Continue to cook until spinach has completely wilted. Remove the pan from the heat.
Heat another non-stick skillet over medium high. Place a tortilla at the bottom. Spread a quarter of the cooked spinach mixture evenly over the tortilla, then add tomatoes on top.
Sprinkle cheddar cheese and feta cheese and cover this with another tortilla.
Once the cheese starts to melt and the bottom tortilla gets crispy (about 2 to 3 minutes), carefully flip the quesadilla to allow the other side to cook.
Once done, remove it to one of your warmed baking sheets and cover loosely with aluminum foil.
Repeat the process for the other 3 quesadillas.
When ready to serve, slice into wedges. Serve with your favorite salsa or guacamole