Start by boiling the russet potatoes. Place them in a large pot and cover with cold water. Add a pinch of salt. Bring the water to a boil, and let it simmer for about 15-20 minutes or until tender. Drain the potatoes and return them to the pot. Use a potato masher to mash them thoroughly.

To the mashed potatoes, add the melted butter, whole milk, and the egg yolk. Mix until smooth and season with salt and pepper to taste. Set this mixture aside.

In a large skillet, heat the vegetable oil over medium-high heat. Once hot, add the chopped onions and sauté for about five minutes until they become translucent. Stir in the garlic, then add the ground beef. Cook until browned, breaking it apart as it cooks.

Next, sprinkle the flour over the browned meat. Stir in vigorously for a couple of minutes to cook out the raw taste. Gradually add the beef broth, stirring continuously, and bring this mixture to a simmer. Add the chopped carrots, mushrooms, and tomato paste. Toss in the Dijon mustard, thyme, bay leaf, and Madeira wine. Simmer for about 10-15 minutes, allowing the flavors to meld beautifully.

In the last five minutes of cooking your meat mixture, add the peas and green scallions. They will add a fresh crunch to the dish. Once everything has combined well, remove the bay leaf. Taste and adjust seasoning with salt and pepper.

Preheat your oven to 400°F (200°C). In a 2-quart baking dish, spoon the meat filling and spread it out evenly. Gently layer the creamy mashed potatoes over the top, smoothing it out with a spatula. Optional: Use a fork to create ridges on the surface; this creates crunchy bits that taste fantastic.

Bake in the preheated oven for about 30 minutes, or until the top is golden and crusty. This is the time to take in the delightful aroma that fills your kitchen.

Remove from the oven and let it rest for a few minutes before serving. This helps with the layering and allows the flavors to settle. Serve alongside your chosen pairings, and relish each satisfying bite.