Cut 6 palm-sized squares of parchment paper and set them aside on a baking sheet.
To a food processor add ½ the black beans, 1/2 the cooked sweet potato and 4 chipotle peppers. Save the remaining peppers and adobo sauce to make a chipotle cream sauce later. Pulse until a thick paste forms, taking time to scrape down the sides in between mixing.
Transfer the mixture to a large bowl. Add the remaining black beans, sweet potato, 1 cup panko, 1 cup cooked quinoa, 1 tsp cumin and 1 tsp garlic powder. Using your hands, give it a good mix, crushing the large sweet potato and black bean pieces into smaller chunks as you go. Add 2 tsp salt and mix again until well incorporated.
Lightly coat your hands in oil, then scoop ½ cup of the mixture into your hands and shape into a large patty, about the diameter and thickness of your palm. Transfer to a parchment square and set back on the baking sheet. Repeat until you have 4-6 patties.
Add 2 tbsp of neutral oil to a large non-stick skillet and heat on medium-high. Once hot, reduce to medium and add burgers, working in batches as needed and using the parchment paper to help transfer without breaking. Cook for 5-7 minutes on one side, or until a crispy, golden brown crust forms. Flip and finish cooking on the other side, until completely warmed through, about 5 minutes.
Set burgers aside and make the chipotle cream sauce by chopping 2 chipotle peppers and transferring them to a small bowl. Add 1 tbsp of the adobo sauce from the can, 1 cup greek yogurt, the juice of 1 lime and a generous pinch of salt. Mix until well incorporated, taste and adjust seasoning.
Assemble your burgers on brioche buns with chipotle sauce, butter lettuce, avocado and sliced red onion.
