Preheat oven to 350 degrees.
Cut fish and seafood if necessary into about 1" pieces so they are fairly uniform. (Shrimp and bay scallops if using can be left whole). Gently squeeze any excess liquid from the seafood using your fist over your sink. Transfer to a paper towel. This is especially important if any of you seafood has been defrosted.
Melt remaining ¼ cup of butter in a saucepan over low heat. Add the onion and allow to cook about 10 minutes until translucent. Add the vermouth and increase the heat to medium high. Cook until no liquid remains. Then sprinkle the flour over the onions and stir to coat the onions.
Take 1 cup of the reserved fennel cream and add to the pan slowly, stirring constantly. Once the fennel cream is smooth add the milk, whisking until it is incorporated and there are no lumps. Bring to a boil and cook a few minutes to thicken. Add the cream cheese and reserved fronds. Stir till well combined.
Spray a 9 X 13" deep baking dish with cooking spray. Distribute the seafood mixture evenly in the bottom of the pan. Add the hot cream mixture and stir to coat the seafood. Spread of pipe the potato mixture over the top.
Bake about 45 minutes. If the potato topping has not browned sufficiently broil 4-5 minutes til it starts to turn brown.
Note: If you serve this in individual ramekins or scallop shells check for doneness after 30 minutes.