My family eats mashed potatoes at every holiday meal, and as a kid I never liked them, except as a bowl for gravy, which I adored. Imagine my surprise when I learned that you could add other things to mashed potatoes to make them good—such as roasted garlic, leeks, and, as in this dish, parsnips.
Place the onion, carrot, and celery in a large saucepan and sauté over medium heat for 10 minutes. Add water 1 to 2 tablespoons at a time to keep the vegetables from sticking to the pan. Add the garlic and cook for another minute. Stir in the rosemary, lentils, bay leaf, and enough water to cover the lentils by 3 inches. Bring the pot to a boil over high heat. Reduce the heat to medium and cook, covered, for 30 minutes.
Preheat the oven to 350° F.
Add the tomato paste to the saucepan and cook for another 15 minutes, or until the lentils are tender. Season with salt and pepper. Remove from the heat, discard the bay leaf and rosemary sprig, and pour the lentils into a 9 × 13-inch baking dish.
Meanwhile, add the potatoes and parsnips to a medium saucepan and add enough water to cover. Bring the pot to a boil over high heat. Reduce the heat to medium and cook, covered, until the vegetables are tender, about 15 minutes.
Remove the potatoes and parsnips from the heat and drain all but ½ cup of the water. Mash the vegetables until smooth and creamy, then season with additional salt and spread the mixture evenly over the lentils.
Bake the casserole for 25 minutes, or until bubbly. Let sit for 10 minutes before serving.
Chef Del Sroufe is co-owner and executive chef of Wellness Forum Foods, a whole-food, plant-based meal delivery and catering service in Columbus, Ohio. He is the author of Forks Over Knives – The Cookbook and Better Than Vegan. Del teaches cooking classes and helps people transition to a healthy plant-based diet.