Vegetarian shepherd's pie

1 ½ lb russet potatoes

3 tbsp unsalted butter, diced

¼ cup freshly grated Parmesan cheese

½ tsp kosher salt

¼ tsp ground black pepper

¼ cup milk or half-and-half

1 large egg yolk

2 tbsp extra-virgin olive oil

1 yellow onion chopped

4 large carrots scrubbed and diced

2 parsnips peeled and diced (or 2 additional carrots)

8 oz baby bella (cremini) mushrooms roughly chopped

8 oz sliced shiitake mushrooms or more baby bella mushrooms

½ tsp kosher salt

¼ tsp ground black pepper

2 garlic cloves minced

2 tbsp tomato paste

2 tbsp all-purpose flour

1 cup vegetable stock or broth

2 tsp low sodium soy sauce

2 sprigs fresh rosemary or fresh thyme

1 dried bay leaf

One 10-ounce bag frozen peas (no need to thaw)

1. Peel and cut the potatoes into 1” chunks. Place them in a medium saucepan and add enough cool water to cover them by 1”. Cover the pot and bring to a boil over high heat. As soon as the water is boiling, uncover, then reduce the heat to a simmer. Cook, adjusting the heat as needed to maintain a gentle, steady simmer, until potatoes are fork tender, about 12 to 16 minutes. 

2. Drain the potatoes and return them to the saucepan. Remove from the heat.

3. While the potatoes boil, make the filling: Coat a 9×9, 8×10, or 11×7 casserole dish with nonstick spray. Place a rack in the center of your oven and preheat to 400℉. 

4. In a Dutch oven or similar large, deep pot, heat the oil over medium-high heat. Once the oil is hot and shimmering, add the onion. Saute for 3 minutes until it begins to soften.

5. Add the carrots, parsnips, baby bella and shiitake mushrooms, salt, and pepper. Cook, stirring periodically, until the vegetables are softened and the mushrooms brown, about 10 to 15 minutes.

6. Add the garlic and tomato paste. Stir continuously for 30 seconds.

7. Sprinkle the flour over the top of the vegetables. Cook, stirring continuously, for 1 minute.

8. Add the broth, soy sauce, rosemary sprigs, and bayleaf. With a rubber spatula or wooden spoon, scrape the bottom of the pan to remove any stuck on browned bits. Bring the liquid to a boil, reduce the heat to low, then let simmer gently, stirring every few minutes, until the mixture thickens into a rich filling, about 8 minutes. 

9. Remove the rosemary and bay leaf. Stir in the peas. Taste and adjust the seasoning as desired. Spoon into the prepared dish, spreading it into an even layer.

10. Scatter the butter pieces over the top of the casserole dish. 

11. Add the Parmesan, salt, and pepper. With a wooden spoon or rubber spatula, stir a few times, until the butter is mostly melted.

12. Take the boiled potatoes from earlier and mash them well with a potato masher or wooden spoon.

13.  In a small bowl or liquid measuring cup with a spout, whisk together the milk and egg yolk. Add to the potatoes. Stir to combine. The potatoes should be creamy and fluffy.

14. Spoon the mashed potatoes over the top of the filling, starting around the edges to create a “seal” (this will prevent the filling from bubbling up). With a spatula, gently spread the potatoes into an even layer.

15. Line a rimmed baking sheet with a piece of parchment paper to catch any drips, then place the casserole dish on top. 

16. Bake the shepherd's pie on the center rack uncovered for 25 minutes, or until the potatoes begin to brown. Let cool for 10 minutes prior to serving.

17. Serve.