Heat 1 tablespoon olive oil in a large skillet over medium-high heat. Add onion and cook until beginning to soften about 2 minutes. Add the garlic, mushrooms, and 1/4 teaspoon salt, and cook until the mushrooms have browned and the liquid has evaporated. Remove from heat, add the mixture to a bowl, and set aside.
In the large skillet, heat another tablespoon of oil over medium heat. Add the beans, parsley, ¼ teaspoon salt, and 1/4 teaspoon black pepper. Cook 3 minutes or until beans are hot. Transfer to a bowl and mash beans with a fork.
Next, return the large skillet over medium heat and add in 1 tablespoon olive oil. Add in the baby spinach and cook until wilted, then remove from heat.
Assemble your quesadillas by spooning the mashed beans, mushroom mixture, spinach, and vegan mozzarella to one half of a tortilla and folding over the other half to sandwich filling. Over medium-high heat, cook for 3-4 minutes or until golden brown. Flip, and cook for 3-5 minutes or until golden brown.
Serve the quesadillas topped with vegan sour cream, guacamole, salsa, chopped fresh parsley, or toppings of your choice.
