Try to get 6 equal-sized medium to large red onions. Remove the first layer of skin. With a knife, just take the bottom of the core end of the onion off, to give it a flat base, and make 2 cuts in a cross-shape in the top, cutting half way down (do not cut right through into quarters). Push some chopped or pounded fresh thyme into these gaps with a good pinch of salt (it's important to get the salt right into the gaps) and a little knob of butter. I prefer to cook the onions in an earthenware dish on a thin layer of sea salt or I put them in with my roast chicken or lamb and they cook quite happily in the same tray. Place in the oven fat 200C/400 F/Gas 6 for 30-35 minutes.
These onions are great with a roast, so tasty and sweet.