Preheat the oven to 200°C, fan 180°C, gas 6. Put the courgette and carrot chips in a bowl with 1 tablespoon of the olive oil, and toss to coat. Spread out evenly on a baking tray in a single layer. Season with freshly ground black pepper, then roast in the oven for 20-25 minutes, turning occasionally, until golden and tender.
Meanwhile, make the salsa. In a small bowl, mix together the chopped tomatoes and one third of the spring onions with the tomato purée and remaining olive oil. Set aside.
Put the beef mince in a bowl, add the remaining spring onions and season with freshly ground black pepper. Mix together with your hands until combined, then shape the mixture into four burgers.
Heat a dry frying pan until hot. Add the burgers and cook for 5 minutes on each side until browned and completely cooked through. Split and toast the burger buns. Put the watercress leaves on the four bun bases, place the burgers on top and add a spoonful of the salsa. Add the bun tops and serve with the courgette and carrot chips.