Finely chop 1 medium onion and sweat it in butter, being careful not to let it color. Sprinkle the onion with 2 tablespoons (30 milliliters) of curry powder and sweat the mixture for a minute or two more, to bring out the flavor of the curry. Add 2 cups (500 milliliters) of canned or fresh coconut milk and 2 cups (500 milliliters) of sauce velouté to the onion-curry mixture, and let the sauce simmer for 15 minutes. Skim off any scum that floats to the top of the sauce. Finish the sauce with ½ cup (125 milliliters) of heavy cream and the juice of half a lemon.