Place beans, rice, pepper, onions, spices, breadcrumbs, and egg in a food processor and blend or pulse until completely combined.
Add 1-2 tablespoons of avocado oil to a large nonstick pan over medium heat. Once hot, use an ice cream scoop to scoop about 5-6 balls of the mixture and form into patties. Place in the nonstick pan, making sure not to crowd the pan (work in batches, if needed). Cook each patty for 6-7 minutes per side, until golden brown and crispy on the outside. Once cooked on both side, add cheese slices on top and cover to steam to the cheese). Once cheese has melted, remove from pan and set aside.
Add 1 tablespoon of avocado to the hot pan along with the red onion slices. Sprinkle with a bit of salt then cover to steam through. Let onions steam for about 5 minutes, then remove lid and sauté for 3 or so minutes.
Now all you have to do is build your burgers – add chipotle mayo to each bun, avocado slices on the bottom bun, then top it with the black bean burger, sautéed onions, tomato slice, butter lettuce, and the other side of the bun. Now eat up, buttercup!