Preparation

• Preheat oven to 325°F (160°C) and set rack to middle position. In a large pot of salted boiling water, cook shells according to package instructions for baked shells, about 3 minutes less than the package's recommended cooking time. Transfer shells to a bowl of cold water to cool slightly, then drain. Drizzle with oil and toss to coat.
• Fill a large bowl with ice water. Poach shrimp in the same pot of pasta water until just barely cooked through, 1 to 2 minutes. Drain, chill in ice bath, then drain again. Chop shrimp into small pieces.
• In a mixing bowl, combine crabmeat with shrimp, scallops, Dijon, mayo, Old Bay, shallot, and parsley. Mix well and season with salt and pepper.
• In a small saucepan, melt 1 ½ tablespoons of butter over medium-high heat. Add flour and whisk to form a paste. Cook, stirring, until raw flour scent is gone, about 1 minute. Gradually whisk in milk, stirring until sauce thickens slightly, then add bay leaf.
• Reduce heat to low and cook, stirring, until sauce is thick enough to lightly coat the back of a spoon, about 3 minutes. Season with salt and pepper and discard bay leaf.
• Lightly grease a baking dish with oil. Spread ½ cup béchamel sauce in an even layer on the bottom. Fill shells with the seafood mixture and place in the dish opening side up, about 18 shells.
• Spoon remaining 1 cup béchamel sauce on top. Melt remaining 2 tablespoons of butter and mix with breadcrumbs. Season with salt and sprinkle over the shells.
• Bake shells until heated through, about 25 minutes. Switch oven to broiler and broil until breadcrumbs are golden, watching closely. Garnish with minced parsley and serve.