Bean Cake Sandwich

Directions

1. 1
   In a medium bowl, coarsely mash beans with a fork and add flour until well combined.
2. 2
   Shape bean mixture into 4 patties.
3. 3
   Melt butter and oil together in a large skillet over medium heat.
4. 4
   Add bean patties, in batches as necessary, and brown on both sides until crispy.
5. 5
   Spread mayonnaise on 2 pieces of white bread.
6. 6
   Place bean patty on bread and top with onions.
7. 7
   Repeat to make 4 sandwiches.