Vegan Shepherd's Pie

Preheat oven to 350 degrees F. Heat oil in a large, heavy pot over medium-high heat. Add mushrooms, onions, carrots and garlic; cook, stirring often, until the vegetables are slightly softened, about 5 minutes. Add tomatoes, broth, lentils, thyme, rosemary and 1/2 teaspoon each salt and pepper. Bring to a boil over medium-high heat; cover and reduce heat to medium-low. Cook, stirring occasionally, until the lentils are tender, about 35 minutes.

Meanwhile, place potatoes in another large pot; add cold water to cover by 1 inch. Bring to a boil over high heat; reduce heat to medium-high and cook, stirring occasionally, until the potatoes are tender when pierced with a fork, about 15 minutes. Drain the potatoes and return to the pot. Add vegan butter and the remaining 1/2 teaspoon salt; mash with a potato masher until smooth.

Transfer the lentil mixture to a 9-by-13-inch baking dish. Top with the mashed potatoes, spreading them over the lentil mixture in an even layer. Sprinkle with the remaining 1/2 teaspoon pepper. Bake until lightly browned and bubbly, about 20 minutes. Switch oven to broil (do not remove the casserole from the oven); broil until the top is golden, about 5 minutes. Sprinkle with chives.