Add the oil to a large rondeau pot over medium high heat.

Place in the ground beef and cook on all sides to sear it until browned and cooked throughout, which takes about 6 to 7 minutes. Stir this frequently.

Set the beef to the side on a plate and then add the onions and leeks to the pot with the rendered meat fat and caramelize the onions over low to medium heat for up to 45 minutes or until very well browned, sweet, and tender.

Next, add in the carrots, celeriac root, and turnips and sauté for 5 to 6 minutes over medium heat just until very lightly browned.

Stir in the garlic and cook it just until it becomes fragrant, which takes about 30 to 45 seconds.

Add back in the beef to the pan and stir it until combined.

Deglaze with the red wine and cook it until au sec or almost gone, which takes about 3 to 4 minutes.

Stir in the tomato paste until the ingredients are coated.

Next, sprinkle in the flour and stir it to combine.

Pour in the beef stock and cook for 1 to 2 minutes or just until thick.

Season with Worcestershire sauce, Dijon mustard, salt, pepper, and thyme. Adjust any seasonings and stirring these in and tasting it. If you like Dijon mustard go with the full 2 tablespoons.

Transfer the beef filling to a deep 13x9 pan. Briefly set it aside.

In a large pot of boiling salted water add in the potatoes and cook for 12 to 13 minutes or until tender.

Drain the potatoes and hand mash them or run them through a food mill until smooth.

Fold in the buttermilk, melted butter, 1 cup of cheese, egg yolks, salt, and ground white pepper until mixed on. Transfer to optional piping bag with a large star or round tip.

Add the potatoes to the top of the pan with the beef filling in any design you want. You can do stars, zig sags, fleurettes, or even just add to the top and smooth it out.

Garnish with a ¼ cup of melted butter and ½ cup of cheese. Bake at 350° for 25 to 30 minutes or until lightly browned on top.

Sprinkle on an optional garnish of chopped parsley and serve.