Herby lamb burgers with beetroot mayo

INGREDIENTS

• 400g lamb mince
• 1 small red onion , ½ grated, ½ thinly sliced
• handful parsley , roughly chopped
• handful mint , roughly chopped
• 1 tsp olive oil
• 3 tbsp mayonnaise
• 2 cooked beetroot , finely chopped
• 4 bread rolls
• couple handfuls watercress