This recipe is part of our fresh spring dinner party menu.
Make a lid by cutting a piece of parchment paper the same size as the top of the pan, and place directly on onions. Cook about 2 hours, turning every 20 minutes, until onions have no color and are soft but not falling apart. If all liquid evaporates, add 1/4 cup water. Remove from heat and let cool in liquid. Discard bay leaf, peppercorns, and zest. Transfer onions to an airtight container; they can be kept refrigerated for up to 1 week.
Note: The onions can be used with other recipes and served in any number of ways. This is one of our favorites: Toast a baguette, spread apricot jam on top, add a heap of onions, and top with slices of prosciutto.