Heat 1 tbsp. of oil in a large skillet over medium heat. While oil is heating, slice your fajita vegetables and set aside.
Add onions to hot the hot oil and sauté, stirring occasionally, for 5 minutes, until onions are fragrant. Add bell peppers and continue cooking for another 3-5 minutes until peppers are tender and slightly blackened on the sides. Stir in Portobello mushrooms, chili powder, cumin, salt, and pepper. Cook for another 2-3 minutes until mushrooms have shrunk about 50% in size. Stir in black beans then remove from heat and set aside.
While your fajita vegetables are cooking, prepare the guacamole. Cut the avocado in half, gently remove the pit, and spoon out the avocado into a large bowl. Add remaining guacamole ingredients and mash together with the back of a fork until you have a smooth, spreadable guacamole.
In a separate skillet, heat up the remaining oil. Add 1 tortilla. Spread 2-3 tablespoons of guacamole on 1/2 of the tortilla. Top with a heaping spoonful of the fajita vegetables (it should be 1/6th of the vegetables). Fold the tortilla over and cook on each side for about 30 seconds until tortilla is nicely browned and crispy.
Once quesadilla is done, remove from heat and set aside. Continue until all the filling is gone. It should make 6 tortillas.
Cut quesadillas in half and serve with cilantro, jalapeños, and hot sauce.