Rustic Lentil & Mushroom Shepherd's Pie

1. Boil Potatoes

Start by boiling the peeled and quartered potatoes in a large pot of salted water. Use 2 tablespoons of kosher salt to season the water. Let the potatoes boil for about 15-20 minutes, or until a knife can pierce them with almost no resistance.

2. Prepare Lentils

In a 10-inch oven-safe skillet with high sides or an enameled cast-iron braiser, simmer the lentils with the thyme, 2 cups of vegetable broth, and 1 teaspoon of kosher salt for about 20 minutes until the lentils are tender but not mushy.

3. Mash Potatoes

Once the potatoes are done, drain them and return them to the pot. Add 6 tablespoons of butter, 1/2 cup of whole milk, 1/2 cup of sour cream, and half of the Parmigiano-Reggiano cheese. Mash everything together until smooth and creamy. Season with salt and freshly ground black pepper to taste.

4. Cook Vegetables

In the same skillet used for the lentils, melt 4 tablespoons of butter and cook the sliced mixed mushrooms for about 8-10 minutes, until they release their moisture and start to brown. Add the thinly sliced leek, diced carrots, and minced garlic, and continue to cook for another 10 minutes until the vegetables are tender. Season with salt and freshly ground black pepper.

5. Add Tomato Paste and Flour

Stir in 2 tablespoons of tomato paste and cook for about 2-3 minutes to deepen the flavor. Sprinkle 2 tablespoons of flour over the vegetables and stir to coat.

6. Combine Filling

Gradually add the remaining 1.5 cups of vegetable broth while stirring, allowing the mixture to thicken. Add the cooked lentils, thawed peas, and fresh lemon juice to the skillet, stirring until everything is well combined and the filling is thickened.

7. Bake Shepherd's Pie

Preheat your oven to 375°F. Spread the mashed potatoes evenly over the filling in the skillet, or transfer the filling to a 3-quart casserole dish and spread the mashed potatoes on top. Sprinkle the remaining Parmigiano-Reggiano cheese over the top, if using. Bake the shepherd's pie in the preheated oven for about 30 minutes, until the top is golden and the filling is bubbling.

8. Let Stand

Let the pie stand for 15 minutes before serving to allow the flavors to meld and the pie to set. Enjoy your hearty and comforting Rustic Lentil & Mushroom Shepherd's Pie!