Dense penne and prosciutto pie with a potato crust is a variation on timballo di maccheroni -- baked macaroni in a drum shape -- traditionally served in many regions of Italy, including Apulia, Basilicata, and Calabria.
Preheat oven to 375 degrees. Bake potatoes on a rimmed baking sheet until tender, about 45 minutes. Let cool slightly. Peel potatoes; discard skin. Mash with a potato masher. Stir potatoes, butter, and scallions in a medium bowl; season with salt and pepper. Press potato mixture evenly into bottom of a 9-inch springform pan. Bake until light golden, 25 to 30 minutes. Let cool on a wire rack.
Bring milk and cream to a boil in a medium saucepan over medium-high heat, stirring occasionally. Remove from heat; stir in chile. Whisk in eggs, egg yolks, sour cream, Parmesan, and nutmeg. Season with salt and pepper; set aside.
Heat oil in a large skillet over medium heat. Add shallot; cook, stirring occasionally, until light golden, about 2 minutes. Add chard; cook, stirring occasionally, until wilted, 3 to 4 minutes. Season with salt and pepper.
Stir pasta, Parmesan mixture, chard mixture, prosciutto, and oregano in a large bowl. Spread over crust. Bake until golden brown and set, about 45 minutes. Let cool slightly before serving.