Preheat oven to 400°F.
Cut puff pastry into 8 (4 to 5 inch) seashell shapes. (See Tip 6)
Place on silicon or parchment lined baking sheet. Using a sharp knife, score top of each shell several times by cutting to, but not through, the bottom of the pastry to form the shell design. Bake in preheated 400°F oven for 20 minutes, or until puffed and golden brown. Remove from oven. Using a sharp knife, slice each shell in half.(See Tip 7)
While shells are baking, melt 2 tablespoons unsalted butter in saucepan over medium heat. Add green onions and green pepper; sauté for 5 minutes until tender. Add mushrooms; continue cooking for 2 to 3 minutes. Stir in pimientos and dry sherry. Cook for 1 to 2 minutes. Transfer to a bowl; set aside.
In the same saucepan, melt remaining 2 tablespoons butter over medium heat. Add flour; stir constantly for 1 minute, or until smooth. Slowly add cream; whisk over medium heat until thickened and bubbly.
Add vegetable mixture to cream sauce, along with dry mustard, nutmeg, cayenne pepper, and salt. Cook until heated.
Add chopped shrimp; cook until heated. Correct seasonings.
Place half of pastry shell on plate; spoon on shrimp newburg mixture; top with other half of puff pastry shell.
Yield: 6 servings.