Preheat oven to 355F/ 180C. Toss rhubarb in a bowl with melted butter and sugar. Transfer to an ovenproof dish and bake for 10 minutes or until rhubarb is soft. Divide among greased ramekins and fill any gaps with raspberries.
Place orange juice, lemon zest and yolks in a heatproof bowl set over a small pan of gently simmering water. Heat, whisking constantly, for 3 minutes or until light and creamy.
Remove bowl from heat, gently fold in whipped cream, then spread mixture over rhubarb and raspberries. Dust with icing sugar and broil them for 2 minutes or until tops are golden. Serve rhubarb and raspberry gratin immediately.