Place the dried plums in a saucepan with 1 cup water and cook at a simmer until soft, about 30 minutes. Push the mixture through a strainer to puree. Add the sugar to the puree and cook 5 more minutes or until it is like marmalade. Let cool slightly. Beat the egg whites until stiff peaks, then gradually mix in the puree, and lemon juice.
Pour into a buttered casserole or souffle dish and bake for 30 to 45 minutes. Serve warm or cold with whipped cream.