Is there anything more comforting than mashed potatoes and stew all baked together in one gorgeous casserole? Of course not. The potatoes are golden and browned on top. Poke your fork in and inside is a juicy concoction of lentils, mushrooms, and veggies. Meat-and-potatoes heaven in one piping hot dish.
2cups (480ml) cold vegetable broth
1yellow onion, chopped into medium dice
2 medium carrots, thinly sliced into half-moons
2ribs celery, thinly sliced
1pound (450g) cremini mushrooms, chopped into pea-size pieces
Chopped fresh flat-leaf parsley, for garnish (optional)
In a measuring cup, mix together the broth and cornstarch until the starch is mostly dissolved. Mix in the tomato paste and set aside.
Preheat a large pan over medium heat. Sauté the onion and carrots in the olive oil with a pinch of salt until the onions are lightly browned, 5 to 7 minutes. Add the garlic and sauté for 30 seconds or so, just until fragrant. Add the celery, mushrooms, and thyme with a bunch of fresh black pepper and the salt. Let cook for about 5 minutes, until the mushrooms have released a lot of moisture.
Add the lentils and use your spatula to mush them in the pan to create a “ground beef” effect. Add the cornstarch-broth mixture and mix well. Let cook until the filling is thick and gravy-like, about 7 minutes. Fold in the peas. Taste for salt and seasoning.
Preheat the oven to 350°F (175°C). Lightly grease an 11 by 13-inch (28 by 33 cm) casserole dish. Transfer the filling to the casserole in an even layer. Dollop the mashed potatoes over the top, starting at the edges and working your way in, and then use a rubber spatula to spread them relatively smoothly.
Place the casserole dish on a baking sheet to collect any juices that bubble over. Bake for 25 to 30 minutes, until the potatoes are browned. Let cool a bit before sprinkling with parsley, if desired, and serving.