I made the Lemon Pavlova, but upon slow addition of sugar I noticed my meringue become runny and flat - all the air from the egg whites was gone. Continuous beating didn't make a difference. I added about a teaspoon of cream of tartar which made all the difference and salvaged the meringue! Did Nigella forget to include it?
Nigella's Lemon Pavlova has a fairly traditional pavlova meringue base that is flavoured with lemon. The sugar is added once the egg whites have been whisked until they are at the point where they form satiny peaks, as with most meringue mixtures. If the meringue mixture becomes flat or runny when the sugar is added then it usually means that the egg whites were not quite whisked enough before the sugar was added. It sometimes helps to whisk the whites, then add a tablespoon of sugar and whisk the whites back to medium peaks before adding the rest of the sugar. This will help to prevent the whites from becoming over-whisked and separating, but allow you to make sure that they have been whisked enough before all of the sugar is added.
Acids are added to meringues to help stabilise the whisked egg whites and to stop the meringue from collapsing before it is baked. Cream of tartar is one form of acid but in this recipe Nigella adds acid in the form of lemon juice. The lemon juice doesn't flavour the meringue, it is the lemon zest that adds the flavour. So acid is added to the meringue and the recipe is not missing an ingredient.