In a bowl, combine ground beef, Worcestershire sauce, salt, and pepper.
Form beef into burger patties.
In a large pan, heat avocado oil over medium heat. Add portobello mushroom caps and cook for about 3-4 minutes on each side. Remove from heat.
In the same pan, cook burger patties for 4 minutes on one side and 5 minutes on the other side, or until desired doneness is achieved. Add cheese to top of burgers and cover with a lid and allow cheese to melt, about 1 minute.
Layer one portobello mushroom cap, then cheeseburger, desired garnishes, and top with remaining portobello mushroom cap.