Preheat oven to 375 degrees F.
Thaw 1 sheet of puff pastry, unwrap it, and brush with beaten egg. Slice pastry into 1-inch wide strips. Weave strips into lattice design, sized to fit the top of a 1 1/2 quart oven-safe medium-sized serving bowl or casserole. Place pastry on a parchment paper-lined cookie sheet and bake until light golden brown. Remove from oven and let stand until needed.
Adjust oven temperature to 350 degrees F.
Melt butter in a medium skillet over medium heat. Add onions, fennel, garlic, celery, carrots, and green peas and saute until tender.
Add scallops and tasso and continue to saute until scallops are barely halfway done. Sprinkle in flour and stir until coated.
Add cream, nutmeg, seasoning, and bouillon. Increase the heat and bring to a boil, stirring constantly. When mixture reaches a boil, remove from heat.
Add crabmeat, shrimp, and oysters and season, to taste, with white pepper. Pour into the oven-safe serving bowl. Top with the pre-baked lattice puff pastry and gently press down into filling. Bake about 10 minutes or until bubbly.
Remove from the oven and let stand for 5 minutes before serving.