Cover apricots with cold water. Add lemon rind. Cover and leave 3 to 4 hours or overnight.
Simmer apricots and liquid in a covered saucepan. Drain, reserving liquid. puree apricots with 120ml (4 fl oz) of cooking liquid.
Preheat oven to 190 C / Gas 5.
Tie an outer collar of greaseproof paper round a 1.2L (2 1/2 pt) souffle dish. Brush inside of dish and collar with melted butter.
Melt butter in a saucepan. Add flour and cook 1 minute before blending in milk. Bring to boil, stirring all the time. Cook for 1 minute. Remove from heat and cool. Stir in apricots, sugar and egg yolks. Carefully stir in remaining cooking liquid.
Whisk egg whites until stiff but not dry. Add one spoonful to mixture and beat in. Lightly fold in remainder. Pour into prepared dish. Level the top and make a few cuts through the outer edges.
Stand dish on baking tray and cook for 35 to 40 minutes. Remove paper and sprinkle with flaked almonds. Serve with single cream with Amaretto added.