French Onion Soup Shepherd's Pie with Gruyere Mashed Potato Topping

Preheat oven to 375°F (190°C).

In a large pot or Dutch oven, heat butter and olive oil over medium heat. Add sliced onions and cook, stirring occasionally, until deeply caramelized, about 30-40 minutes. This is crucial for the flavor, so be patient and allow the onions to develop a rich, brown color.

Stir in flour and cook for 1 minute. Deglaze the pot with dry sherry (if using), scraping up any browned bits from the bottom. If not using sherry, add a splash of vegetable broth and scrape the bottom.

Add vegetable broth, bay leaf, thyme, and Worcestershire sauce. Bring to a simmer, then reduce heat and simmer for 15 minutes to allow the flavors to meld. Season with salt and pepper to taste. Remove the bay leaf.

While the soup is simmering, prepare the mashed potatoes: Place potatoes in a large pot and cover with cold water. Bring to a boil and cook until tender, about 15-20 minutes.

Drain the potatoes and return them to the pot. Add milk, butter, and half of the Gruyere cheese. Mash with a potato masher or electric mixer until smooth and creamy. Season with salt, pepper, and nutmeg.

Pour the French onion soup into a 9x13 inch baking dish.

Carefully spread the Gruyere mashed potatoes evenly over the soup.

Sprinkle the remaining Gruyere cheese over the mashed potatoes.

Bake in the preheated oven for 20-25 minutes, or until the topping is golden brown and bubbly.

Let cool for a few minutes before serving. Garnish with fresh parsley.