Purée of Onions

• ¾ kg (1½ lb) sliced onions
• 4 tablespoons unsalted butter

Blanch the sliced onions in a large quantity of boiling water for 3 minutes. Drain, then place in a heavy saucepan with half the butter. Stir for a few minutes to coat the onions in the fat then add the cassis. Stir in the rice then pour over the stock, add salt and pepper and bring to a boil. Reduce the heat and simmer gently, uncovered, until the rice is cooked. Purée in a processor or food m