In a saucepan, combine the onions, lemon juice, tomatoes, salt, cayenne, garlic, and wine over medium-high heat and bring to a boil. Reduce the heat to medium and simmer until vegetables are soft, about 12 minutes.
Add the cream and simmer for about 4 minutes or until the sauce thickens. Stirring constantly, add half of the butter, a little at a time. Remove from the heat and, stirring constantly, add the remaining butter. Add the parsley. Serve immediately.
Yield: 1 3/4 cups