Cook pasta shells in salted boiling water until just under al dente. Drain, cool in cold water, and toss with olive oil.
Poach shrimp 1–2 minutes in the same water, transfer to ice bath, drain, and chop.
Mix chopped shrimp, crabmeat, scallops, Dijon, mayo, shallot, parsley, Old Bay, salt, and pepper in a bowl.
In a saucepan, melt 1.5 tbsp butter. Whisk in flour for 1 minute. Gradually whisk in milk and bay leaf. Simmer until thickened, then season and discard bay leaf.
Spread ½ cup béchamel in a greased 9x13-inch baking dish. Fill shells with seafood mix and arrange in dish.
Top with remaining béchamel. Mix panko with 2 tbsp melted butter and sprinkle over shells.
Bake at 325°F for 25 minutes. Broil briefly until topping is golden. Garnish and serve.
