Preheat oven to 450 degrees F. Pierce potatoes with a fork, place in a plastic bag and coat with oil. Bake directly on an oven rack for 1 hour, or until tender when pierced with a skewer. Remove potatoes from the oven. While hot, cut off 1/4" of the top. Scoop out the flesh into a bowl, being careful not to tear the skins. Rice the flesh.
Whisk together half & half, parsley, scallions, Parmesan, Dijon, mayonnaise and egg yolk in a large bowl; set aside.
Saute onions, peppers and garlic in oil in a non-stick saute pan. Add butter and the next 4 ingredients and cook for 1 minute. Stir in the shrimp and crab. Cook until shrimp is pint, 2-3 minutes, stirring frequently. Add mixture to the cream and egg mixture, then fold in riced potatoes Season with salt.
Stuff potato mixture into the potato shells using a spoon. Mounding the tops. Arrange stuffed potatoes on a baking sheet, then top with cheese. Bake at 450 degrees F until golden brown, about 10 minutes.
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