How to Make It
Preheat oven to 450° F with the racks in the upper and lower thirds.
Roast the sausage and potatoes on a large rimmed baking sheet on the bottom rack until the sausage is cooked through and the potatoes are nearly tender, 20 to 25 minutes. Add the mussels, shrimp, butter, and 1 teaspoon of the seafood seasoning to the baking sheet and toss to combine. Roast until all the mussels have opened and the shrimp are opaque, 12 to 15 minutes more.
Meanwhile, roast the clams on a separate large rimmed baking sheet on the top rack until opened, 10 to 15 minutes. (Discard any mussels or clams that do not open after 15 minutes.)
Toss the clams with the sausage, potatoes, mussels, and shrimp and sprinkle with the parsley. Serve with extra seafood seasoning on the side.