Moray Fish Fillets

1-2 lb/0.5kg-1kg fish fillets (smoked or white fish may be used)
12 oz (350g) long grain rice, cooked
12 oz (350g) onion, chopped
1 oz (25g) butter
1 1/2 oz (40g) flour
1 pt (600ml) milk
8 oz (225g) cheese, grated
2 tsp curry powder
salt and pepper
1 tblsp lemon juice