Preheat the oven to 200°C, fan 180°C, gas 6. Toss the sweet potato wedges with 1 tablespoon of the oil then tip into a large roasting tray. Season with freshly ground black pepper and bake for 35 minutes, turning halfway through, until crisp on the outside and tender on the inside.
Meanwhile, in a large bowl, mix together the mince, onion chutney and egg until well combined. Using wet hands, divide and shape the mixture into 4 burger patties.
Heat a griddle pan or barbecue to high. Brush the burgers with the remaining oil and cook for 4-5 minutes on each side, until they are cooked through with no pink remaining. Remove from the heat and set aside to rest for a few minutes.
Cut open the rolls and top the bottom halves with a few salad leaves, the burger patties, tomato slices and a little mayonnaise. Top with the remaining roll halves.
Combine the chopped tomato, cucumber and remaining salad leaves in a bowl and serve with the burgers and sweet potato wedges.