Heat a cast iron skillet. When hot, add oil, swirl to coat bottom.
Add bacon, cook over medium heat until almost crisp. Remove to a paper towel lined plate; keep warm.
Pour off all but 2 teaspoons oil.
Add onion rings to the skillet, toss to coat with oil. Reduce heat and cook until limp. Add, in its own little pile, to plate with bacon.
Add mushrooms to skillet, toss to coat with oil. Cook until well browned, then salt to taste. When tender, add to bacon and onions, in its own little pile on the plate and keep warm.
Increase heat, add pineapple slices to the skillet. Cook until slightly charred, turning to brown both sides. Add to the plate and keep warm.
Add veggie burgers to the skillet. Cook until browned, basting occasionally with reserved pineapple juice. Turn and brown other side, basting with pineapple juice. Make sure they are cooked through (see package for temperature, most will say 160F).
Top burgers with cheese; cover pan briefly to melt cheese.
Serve burgers on buns with barbeque sauce, a bacon slice, a pineapple slice, tomato, lettuce, onions and mushrooms.