1. Preheat oven to 400F.
2. Boil the potatoes: Place the halved potatoes in medium-sized pot. Cover with an inch of cold water. Add a pinch of salt. Bring to a boil, reduce to a simmer, and cook until tender. This will take about 20 minutes.
3. Lentil Vegetable Mixture: Saute the onions, leeks, garlic, and herbs in a large-deep pan (or dutch oven) with a splash of water until the onions and leeks are tender.
4. Add the mushrooms and carrots to the pot. Add a dash of salt and pepper. Cook until the mushrooms reduce to half their size, adding a splash of water when necessary to the pan so the vegetables do not stick to the pan.
5. Add the frozen peas, red lentils, Worcestershire sauce, bouillon, and water to the pan. Bring to a simmer, and cook until the lentils are cooked (about 8 to ten minutes).
6. Stir the vegetable mixture and blend 1/3 of the vegetable mixture with an immersion blender (this will give it that perfect, hearty texture). Taste test and add additional salt, pepper, and herbs to taste.
7. Mash the cooked potatoes: Cook the potatoes until the are easily pierced with a fork. Drain the potatoes and place them back in the pot. Add 2 tbsp of soy milk, garlic powder, sea salt, and pepper. Lightly blend the potatoes with an immersion blender until the potatoes are fluffy and thoroughly mashed. Add a splash more soy milk if necessary.
8. Assemble the casserole: Spread the lentil vegetable mixture in an even layer in a large baking dish (8×13 casserole dish). Place the mashed potatoes on top. With a damp spatula, smooth out the mashed potatoes on top. Place the fresh rosemary on top of the potatoes.
9. Bake for 40 minutes until golden brown. Leave to cool for ten minutes before serving.

Instant Pot Instructions:

1. Mashed Potatoes: Add the potatoes and 2 cups of water to the Instant Pot. Pressure cook on high for 13 minutes and then quick release until the steam releases.. Drain the potatoes and transfer to a separate large bowl to mash. Follow the rest of the instructions above.
2. Lentil Vegetable Mixture: After making the mashed potatoes, sauté the vegetables as per the instructions above in the Instant Pot on the high setting.
3. Add the frozen peas, red lentils, Worcestershire sauce, bouillon, and water to the Instant Pot. Pressure cook on high for 3 minutes and use the quick release until the steam releases. Follow the rest of the instructions above.