FOR THE CLAM BROTH:
1. Pour water into a casserole. Add onion, celery, bay leaf and peppercorns, and let boil.
2. After 2 minutes, add the clams and remove from heat as soon as the clams open. Strain and reserve the broth for the sauce. Remove clam meat from shell, and reserve.
FOR THE SAUCE:
1. Melt butter in a saucepan, and add the flour, whisking continuously.
2. After 3 minutes, add the clam broth and continue whisking until sauce thickens.
3. Add milk and then cream, and simmer until desired consistency is achieved. Remove from heat.
FOR THE FISH:
1. Season fish fillets with calamansi juice, salt and pepper.
2. Arrange a layer of potato slices over each fish fillet, slightly overlapping in a manner similar to fish scales.
3. In a saucepan over moderate heat, sear fish fillets with potato scales for about 2 minutes. Turn fish fillets over and sear the other side for about 1 minute. Transfer to a tray.
4. Arrange fish fillets in an oven-proof dish, garnish with the reserved clams and asparagus. Pour enough sauce over fish, sprinkle with chopped parsley and Parmesan cheese.
5. Bake in a 350°F oven for about 15 minutes or until the top is golden brown.