Vegetarian Shepherd's Pie with Mushrooms

Preheat the oven to 375 degrees F (190 degrees C). Grease 9x13-inch baking dish.

Heat a large pot of water over medium-high heat and bring to a boil. Drop potatoes and 1 teaspoon of salt into the water. Boil until the potatoes are very tender, about 20 minutes. Drain potatoes, reserving 2 cups of the cooking water (potato stock).

Mash potatoes in a bowl with milk and 3 tablespoons butter. If potatoes are too dry, add some of the potato stock gradually until mashed potatoes are light and fluffy. Season with salt and pepper and set aside.

Place 2 skillets on the stove. Add 1/4 cup potato stock, 1 tablespoon butter, and 1/2 of the garlic to each skillet and heat over medium heat until butter is melted. Place 1/2 of the mushrooms in each skillet and cook, stirring occasionally, until mushrooms are soft, about 10 minutes, adding more potato stock if mushrooms become too dry. Remove from the heat and transfer mushrooms and any cooking liquid to the prepared baking dish.

Melt remaining 2 tablespoons butter in one of the skillets over medium heat. Add onions and peas; cook until onions are soft and translucent, about 5 minutes. Layer onion and peas on top of the mushrooms. Spread mashed potatoes on top.

Bake in the preheated oven until the top is lightly browned, about 15 minutes.