Light your barbecue or heat a griddle pan over a low heat. Whisk the honey, lime juice and ½-1 tsp chilli flakes (they’re spicy, so use as much or as little as you like) with a pinch of salt and pepper, to make a smooth dressing. Set aside.
Toss the walnuts into a dry frying pan and toast on a low heat for 3-4 mins till fragrant and a little browned. Roughly chop. Set aside in a bowl.
Trim, peel and thinly slice the shallots. Trim and dice half the cucumber (the other half will keep in the fridge for a few days. use it in salads and sandwiches). Shred the mint leaves.
Halve the watermelon. Chop half into 2cm-thick slices, then cut into quarters. Leave the skin on.
If you’re using a griddle pan, brush it with 1 tbsp oil. Thickly brush one side of the watermelon slices with half the honey dressing. Place, honeyside down, on the barbecue or griddle. Cook for 3–5 mins till charred. While the watermelon cooks, brush the top side with more of the dressing. Flip the slices over and cook for another 3–5 mins.
Stack the watermelon slices on a serving plate. Scatter over the walnuts, cucumber, shallots and mint. Crumble over half the feta. Drizzle with any remaining honey dressing. Serve straight away.