1 cup (7 oz.) dried brown or green lentils, picked over and rinsed
2 1/2 C. water
One (1-lb.) head cauliflower
1 lb. Yukon Gold potatoes, scrubbed and cut into 1-inch cubes
1/4 C. unsalted vegan or dairy butter
1 tsp. ground turmeric
1 1/2 tsp. kosher salt, divided
2 T. extra-virgin olive oil
1 large onion, chopped
2 carrots, diced
1 pound cremini mushrooms, quartered
2 T all-purpose flour
1/2 tsp. freshly ground black pepper
2 tsp. chopped fresh thyme leaves
2 T. tomato paste
1 C no-salt-added vegetable broth
2 T. red wine vinegar
1 C. frozen corn
Position the rack in the middle of the oven and preheat to 400 degrees.
In a small saucepan over medium-high heat, combine the lentils and water. Bring to a boil, reduce the heat, cover and simmer until the lentils are tender, 20 to 30 minutes. Drain.
While the lentils are cooking, cut the cauliflower into florets and remove the core and large stem. Set up a steamer basket over medium-high heat, add the potatoes and cauliflower and steam until very tender, about 25 minutes. Transfer to a large bowl, add the butter, turmeric and 1/2 teaspoon of the salt and mash with a potato masher until smooth and fluffy.
In a large skillet over medium heat, heat the oil until shimmering. Stir in the onion and carrots and cook, stirring, until they start to soften, 8 to 10 minutes. Increase the heat to medium-high and stir in the mushrooms. Cook, stirring occasionally, until the mushrooms soften and exude their liquid and it mostly evaporates, about 10 minutes. Stir in the flour, the remaining 1 teaspoon salt, the pepper and thyme and cook, stirring, until fragrant, about 30 seconds.
Whisk the tomato paste into the broth and stir it into the mushroom mixture, along with the cooked lentils. Stir in the vinegar and bring the mixture to a gentle boil, then turn off the heat and stir in the corn. Transfer the mixture to a 9-by-13-inch baking dish.
Spread the cauliflower mixture on top of the lentil-mushroom mixture in the dish and bake 25 to 30 minutes, until the filling is bubbly and the topping starts to brown on the edges. Serve hot.