Pack whole tomatoes into quart/litre jars with a few thyme sprigs, a couple of bay leaves, and an onion slice or two. Close the lids and set the jars on a rack in a deep pan. Add enough water to cover generously. Weight down with a brick so they don't float. Simmer for an hour and a half, until they lose their shape and collapse. Let the jars cool in the water so that they form a tight seal. When done, each jar will look only half full of tomatoes.