Bring to a boil, reduce heat to medium, and cook until potatoes and parsnips are fork tender, about 15 - 20 minutes. Remove from heat and drain.

Combine potatoes, parsnips, 2 Tablespoons butter, 1 Tablespoon parsley, cream, salt and 1/4 teaspoon pepper in a large bowl. Beat with a hand mixer until smooth and creamy. Set aside.

Heat olive oil in a large skillet over medium-high heat. Add the onions and cook until they begin to soften, about 4 minutes. Add the meat and cook until browned, breaking it apart with a wooden spoon, about 8 minutes. Drain fat.

Preheat oven to 350 degrees.

Return the meat to the skillet. Stir in the flour and cook over medium-high heat, stirring until flour is golden brown, about 2 minutes.

Add the tomatoes, thyme, sage, 1/4 teaspoon pepper, and remaining 1 Tablespoon of parsley. Simmer for about 3 minutes.

Stir in the beef stock and cook, stirring constantly, until the entire mixture starts to thicken, 3 - 4 minutes.

Pour into a 9" deep-dish pie pan.

Scoop potato mixture into a large zipper bag (or decorator bag) fitted with a 1" fluted tip.

Pipe the potato mixture over the meat. Start in the center and pipe in a circular motion until the entire pie is covered with potatoes. Drizzle 1 Tablespoon of melted butter over the top and bake until the potatoes are golden brown, about 35 minutes.