Halve the red onions through the core, keeping the skin on. Brush the cut faces with oil and place cut-side down on a hot grill. Roast for about 12 minutes till the cut surface is black and charred, then flip to skin-side down and leave to cook for 5 minutes more. Remove to a plate to chill till they are cool enough to handle.
Make the dressing while you wait. Roast the walnuts over the fire for about 8 minutes in an old sieve or dry frying pan, stirring occasionally. Slightly crush them and mix with all the other dressing ingredients apart from the mint leaves.
Break the cooled onion halves into petals, discarding the outer skins. Set the petals with their charred rims upwards on a large serving plate. Just before serving, thinly shred the mint leaves and mix into the dressing. Drizzle all over the onion petals and sprinkle with a little sea salt to finish.
To cook without a BBQ:
– Use a lightly oiled, preheated griddle pan on your stove top and cook just as you would on the fire, but beware, your house will get pretty smoky!