Main Ingredients Potato Topping

1. Dice the potatoes into 1-inch cubes. Add the potatoes to a large pot and cover them with water. Bring the water to a boil and then simmer it until the potatoes are tender, which takes about 15 minutes. Drain the potatoes and then add the butter or oil. Mash the butter or oil into the potatoes until it melts. Then, add the milk and stir until the potatoes reach the desired consistency. Add salt and pepper to taste and then set the potatoes aside in a warm place.
2. Set your oven to 200 degrees Celsius.
3. Heat the oil or water in a shallow pot or Dutch oven over medium heat. Add the onions, carrots, celery, and garlic and sauté them for 5 minutes. Add the thyme and rosemary and cook for 1 more minute. Stir in the lentils, tomato paste, Worcestershire sauce, and flour and stir until the flour is incorporated and no white streaks remain. Add the corn, peas, Dijon mustard, salt, pepper, and vegetable broth and simmer for 5 minutes, stirring occasionally. Taste the mixture and add more salt and pepper to season it to your taste.
4. Evenly spread the mashed potato topping over the vegetables in the pot you cooked them in or transfer the mixture to a 9x9 or 9x11 baking dish. Spread the potatoes from edge to edge. You can leave the potatoes as they are or use the back of a fork to create a criss-cross pattern in the potatoes by dragging the fork from one edge to the other and then in the opposite direction.
5. If your baking dish is very full, place it on a rimmed baking sheet so that the filling doesn't bubble over into your oven. Place the baking dish in the oven and bake it, uncovered, for 25 to 30 minutes. Allow the dish to cool for 15 minutes before serving.
6. Store leftovers in the refrigerator in a covered container for up to 5 to 6 days. For longer storage, keep the leftovers in the freezer for 2 to 3 months. See the notes for freezer storage instructions.