Beat egg yolks until creamy and gradually stir in 200 g sugar.
Add lemon juice and grated lemon zest. Beat egg whites until stiff and gently fold into egg yolk mixture. Divide mixture among 4 ramekins lined with melted butter and sprinkled with sugar and cook in a water bath for 30 minutes.
For the sauce, melt the sugar and caramelize lightly. Add orange juice, lemon juice and orange liqueur and boil down, add the butter. Pour boiling water over the orange peel, then cut into thread-thin strips and garnish the lemon mousse with them.