In a large skillet, heat 1 tablespoon of olive oil. Saute the mushrooms over medium-high heat until they’ve browned and released their liquid, 3 minutes. Season with salt and remove to a bowl.

Add 1 tablespoon olive oil to the pan along with the onion, carrots, and leeks and sauté until soft, 5 minutes. Stir in the garlic and chili flakes and cook one minute more. Fold in the lentils, tomatoes, cider vinegar, and 1 teaspoon salt. Cook, stirring occasionally, until the tomatoes are soft, 7 minutes. Pour in enough stock to just submerge the lentils (about 2 cups, depending on the size of your pan). Bring to a boil, then simmer on medium-low, stirring every 5 to 10 minutes, until the lentils are al dente, about 30 minutes.

Meanwhile, place the potatoes in a medium stockpot and cover with cold water by 2 inches. Season generously with salt and bring to a boil. Simmer the potatoes until tender, about 15 minutes. Drain and return to the pot. Add 2 tablespoons olive oil and ¼ cup of stock. Mash until the potatoes are smooth and fluffy, adding more stock as necessary to thin them to the desired consistency. Set aside while the lentils are finishing up.

When the lentils are cooked, fold in the mushrooms and taste for seasoning. Transfer to individual ramekins or a medium (8 x 8 or comparable) casserole dish. Top with the potatoes.

Cook under the broiler for 2 minutes to reheat and/or get a nice crust, or simply serve right away.