Put the onion and garlic into a food processor and blitz until roughly chopped. Add the lamb, pesto, mint, if using, and a generous amount of salt and pepper. Pulse until just combined. Be careful not to over process it – you want it to keep some texture. (Alternatively, you can do it all by hand: just finely chop or grate the red onion and garlic and mix everything together.)
Divide the mixture into 4 and shape into burgers. If you have time, pop them on a plate and chill them in the fridge for half an hour before cooking. You can even do this stage the day before and keep them in the fridge overnight.
Fire up the barbie or preheat a griddle pan or the grill, and cook the burgers for 4 to 5 minutes on each side, until nicely browned and cooked through.
Serve in toasted burger buns with a handful of rocket and some sliced tomatoes.