Heat the oil in a frying pan over a medium heat and cook the onion for 5 minutes until soft. Set aside to cool.
Put the mince in a bowl and season with freshly ground black pepper. Add the onion and lemon zest and mix well. Shape into 4 patties.
Cook the burgers in a griddle or frying pan for 5-7 minutes on each side, or until cooked through and no pink remains. Meanwhile, mix together the salsa and pepper.
Lightly grill the cut side of the rolls. Fill the rolls with a burger, a few rocket leaves, and a spoonful each of salsa and houmous.