Try to get 6 equal-sized medium or large red onions. Peel away the outermost layer of onionskin. Carefully slice off the core end of each onion to create a flat base. Make two cuts at right angles through the top of the onion, cutting only halfway through.
Push some chopped fresh thyme and kosher salt down into the cuts, along with a tablespoon of soft butter for each onion.
Cook in an earthenware dish, or in a roasting pan on a layer of kosher salt or sea salt. Place in a preheated 400F oven for 30-35 minute.
You can also tuck the onions into the roasting pan with your chicken or lamb as it roasts.