1/2 cup mayonnaise
1 Tbsp. adobo sauce from a can of chipotle chiles in adobo
1 Tbsp. olive oil
1 medium yellow onion, finely chopped
1 1/2 tsp. kosher salt, divided, plus more to taste
3 garlic cloves, minced
1 tsp. smoked paprika
1/2 tsp. chili powder
1 x 8 oz. block cheddar cheese, shredded
1 x (15 oz.) can black beans, drained and rinsed
1 large egg, beaten
1/2 cup panko breadcrumbs
4 whole wheat buns, split
Unsalted butter, for buns
Shredded lettuce, sliced red onion, and sliced tomato (for serving)