Preheat the oven to 375˚. Line a 9-inch pie plate with the pie dough and crimp as desired; chill 30 minutes. Line with foil, then fill with pie weights or dried beans. Bake until the crust is set, about 20 minutes. Remove the foil and weights and continue baking until lightly golden, 5 to 10 more minutes. Let cool while you prepare the filling.
Make the filling: Combine the roasted peppers and scallions in a large bowl. Add the sausage and cheddar and toss to combine.
Whisk the eggs, heavy cream, 1/2 teaspoon salt and 1/4 teaspoon pepper in a large bowl. Stir in the parsley. Spread the vegetable-cheese mixture in the crust, then pour the egg mixture on top. Reduce the oven temperature to 350˚ and bake until the filling is set, 40 to 50 minutes. Let cool at least 30 minutes before slicing.