Inspired by the classic tortilla de patatas, this tortilla was made with caramelised onions, potato, celeriac, and parsnip with a sprinkling of grated nutmeg. When cooking for lots of people I do this in an oiled baking tray. Just cook the sliced vegetables first separately, a mix with the cooled onions and eggs and bake at 350F until the egg is cooked through. Brown it under the broler if you need to.
