Substitute orange or lemon juice for the lime juice if you wish.
Whisk egg whites, egg, sugar and lime juice in a medium nonreactive saucepan. Add butter and cook over low heat, whisking constantly, until thickened, 5 to 7 minutes. Do not let sauce come to a simmer.
Strain sauce through a fine sieve into a bowl. Place a piece of plastic wrap directly on the surface and refrigerate until chilled.
Make Ahead Tip: Cover and refrigerate for up to 2 days.
1 other carbohydrate, 1/2 fruit