Preheat the oven to 200 degrees. Thinly slice the mushrooms. Mince the the garlic. Chop the parsley. Finely dice the tomato. Zest and halve the lemon.

Heat a large pan over the medium heat. Melt 1 tablespoon butter in the pan, then add the mushrooms. Cook, tossing, for 5-6 minutes, until golden. Add the garlic and a pinch of parsley and cook for another 30 seconds, until fragrant. Season with salt and pepper. Set aside.

Halve and pit the avocado. Scoop out the flesh into a small bowl with a spoon and mash with a fork. Add the tomato, 1 tablespoon chopped parsley, 1 tablespoon lemon zest, and squeeze of lemon juice. Season with salt and pepper and set aside.

Wipe the pan clean and heat over the medium-low. Place 1 tortilla in the pan, then top with a sprinkle of cheese and 1/4 of the mushrooms on one side of the tortilla. Top with another sprinkle of cheese and fold the tortilla over to seal it and allow cheese to melt.

Once golden brown, flip to crisp on the other side. Then, place the quesadilla on a baking sheet and put it in the oven to keep warm. Continue building the other quesadillas, transferring into the oven as you finish.

Cut each quesadilla into wedges and serve with the guacamole to the side.