For the lamb, mix together the oil, herbs, lemon zest and juice, garlic and chilli flakes. Reserve half of the marinade in a separate bowl. Rub the lamb all over with the remainder, place the lamb in a bowl in the fridge and allow to marinade for at least one hour, ideally overnight.
Add the honey and chilli sauce to the remaining marinade – this will become a dressing - and set aside.
For the feta cream, blend the feta with the yoghurt and garlic in a food processor, add the mint leaves and pulse until chopped. Season to taste with salt and pepper and set aside.
To complete the lamb, slice the watermelon and arrange on four plates and scatter with orange segments and red onion (squeeze any orange juice from cutting the segments into the dressing).
Heat a griddle pan, season the lamb fillets and cook them to your liking, turning regularly (6-7 minutes for medium rare). Set aside, in a warm place, to rest.
Drizzle some feta cream over the watermelon, slice the lamb thinly and arrange it over the melon. Scatter with mint leaves and drizzle with the lemon-chilli dressing.