Slowly melt the butter in a frying pan over low heat, add the sifted flour and cook while making sure it doesn't burn. (See here for more instructions: https://cookpad.com/en/recipes/150056-easy-basic-white-sauce)
After turning off the heat, add warmed milk in 2 turns to step one, and briskly whisk it all together with an eggbeater.
Add the bay leaf and nutmeg to Step 2, stewing it at a low heat for about 5 minutes, and then flavor it with fresh cream and consommé stock granules.
Heat up the butter and olive oil in a frying pan, and add the shrimp and scallops in white whine after frying the onions; add salt and pepper and evaporate the alcoholic content.
Mix rice and the white sauce from Step 3 together in a bowl (about 4 ladles' worth).
Add it to the frying pan with the remaining white sauce from Step 4, and bring it to a boil after mixing it thoroughly.
Add rice, sauce, cheese, and parsley in that order to the gratin pans and cook at 200℃ (about 392℉) for 10 minutes.
This is the exquisite sauce and seafood gratin I made: https://cookpad.com/en/recipes/145207-seafood-gratin-with-homemade-sauce