In a frying pan, pour oil and add onion, garlic, spinach, and mushrooms. Add beans and cook on low/medium heat for 5 minutes.
Remove from frying pan and mix until the mixture is smooth, adding breadcrumbs as you mix.
Preheat your Oster® grill to 400°F.
Form mixture into patties approximately 2 ½ inches in diameter. Add burger patties and sliced onions to the grill.
Cover; cook for 5 minutes and flip the onions. After an additional 5 minutes, flip the burger patties. Once onions and patties are fully cooked, remove them from the grill.
Place burger buns onto the grill face down, if desired.
Assemble the burgers onto the buns and top with tomato, lettuce, grilled onions, and cheese (optional).