Boil water, 1 teaspoon of salt and oil in a large sauce pan or dutch oven. Gradually add shells when water is at a rapid boil. Boil for 1 minute less than package directions for "al dente". Shells will continue to cook when placed in the oven. To avoid mushy pasta, you want to initially under-cook.
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 butter in medium saucepan. Add chopped onion and mushrooms. Saute for 5 minutes. Remove mushroom mixture and set aside.
In the same saucepan add flour and whisk into the remaining butter to make a roux (paste). Blend until smooth and bubbly. Whisk in 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 with 2 beaten egg yolks. Whisk together fully and then slowly add to the rest of the sauce. Whisk again. Sauce will get thicker and thicker. Stir in Worcestershire sauce and sherry. Bring to a low boil. Remove from heat.
In a medium mixing bowl, toss shrimp, crab meat, mushroom mixture, parsley and 3/4 cup cream sauce. Stuff mixture into shells. Arrange in a casserole dish sprayed with cooking spray.
Ladle the remaining sauce over stuffed shells. Top with Parmesan cheese.
Place casserole dish in a cold oven and heat to 350 degrees for a total of 20 minutes.
Remove after 20 minutes and serve immediately. Enjoy!
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