Preheat the oven to 350°F and coat a baking or casserole dish with cooking spray.
Cook the shells according to package directions, but for 1 minute less than called for "al dente". Remove and drain on a paper towel lined baking sheet. Dampen 2 paper towels and place them on top of shells to keep them moist. Set aside.
Melt the butter in medium saucepan. Add the onion and mushrooms, saute for 5 minutes. Remove and set aside. Do not wipe out the saucepan.
Add the flour and whisk into the remaining butter to make a roux (paste). Blend until smooth and bubbly. Whisk in the milk, half and half, white pepper and salt. Simmer for an additional 5 minutes, whisking occasionally.
Ladle 1/2 cup of hot sauce to a small dish, while whisking, add the egg yolks. Whisk together fully and then slowly add to the rest of the sauce. Whisk again. Sauce will thicken.
Stir in the Worcestershire sauce and sherry. Remove from the heat and set aside.
In a mixing bowl, toss the cooked seafood with the mushroom mixture, parsley and 3/4 cup of the cream sauce. Stuff the mixture into shells. Arrange in the prepared casserole dish.
Ladle the remaining sauce over stuffed shells and top with the Parmesan cheese.
Bake for 10 minutes, or until cheese has melted.
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