Rinse and drain black beans. Finely chop red onion, tomato and cucumber and combine with black beans.
Heat oil in a medium frying pan. Add burgers and cook on medium-high for 2-3 minutes on each side or until cooked through. Sprinkle with chilli flakes. Wipe out pan with kitchen towel. Halve and toast brioche buns in the frying pan.
Top buns with rocket leaves, burgers, sauce and salsa.