Folding is the key to a high-rising pudding. The goal is to combine the beaten whites into the heavy purée without deflating the whites. One trick is to mix in some of the whites to lighten the purée, then fold in the rest.
Stirring in Purée
Cook the cream sauce until thickened and bubbly. Add the carrot purée, corn, and parsley. Stir with a wooden spoon until sauce and vegetables are thoroughly blended.
Making Stiff Peaks
Put the 3 egg whites in the bowl of an electric mixer. Beat at high speed until the peaks stand up straight when the beaters are removed.
Folding Together
Cut down through the center with the edge of a rubber spatula, coming across the bottom of the bowl, then lifting up in one smooth motion. Rotate the pan a quarter turn for each fold.