2. Make the filling: Preheat oven to 350 degrees. Roll out chilled dough and press into a pie dish. Sprinkle instant mashed potato flakes into bottom of pie shell. (This will help absorb excess liquid from tomatoes.) Next, layer in sliced onion and tomato wedges and sprinkle with salt and pepper. Combine parsley and pimento cheese and spread onto tomatoes. Bake until brown and bubbly on top, 40 to 45 minutes. Cool to room temperature before slicing and serving.