Take a large skillet and put some cooking oil in it. Now, cut the onions and sauté them along with butter in the oil until they become soft. Add broth and clam juice and boil the mixture. Then, add crab, shrimp and scallops, pepper. Cook until the shrimps turn pink. Take the mixture out and keep it separately.
Now, take a saucepan and melt some butter in it. Put some flour in the saucepan and stir until it becomes smooth. After that, add milk and cooking liquid slowly. Then, add salt and pepper according to your taste.
Remove the saucepan from the stove and mix cream, cheese and white sauce in the mixture. Then, take a baking dish and spread the seafood mixture in it. You can use layers of noodles, sauce and cheese on it.
Place the dish in the oven at 350 degrees for around 30 to 40 minutes until it is brown. Take the dish out of the oven and keep it for aside for around 15 to 20 minutes to cool down before serving.