Preheat the oven to 425 degrees F.
Put the lentils and water in a large pot, and bring to a boil. Reduce heat to a simmer, and cook until the lentils are tender, about 20-25 minutes.
While the lentils cook, you can cook the other veggies. In a large skillet, cook the onions, garlic, sliced carrots, and mushrooms in the vegan buttery spread. Cook over medium heat, stirring often, until they are tender. This should take about 15-20 minutes.
If you have not prepared the mashed potatoes ahead of time, you can do that while everything is cooking. Peel and slice the potatoes, and put them in a pot with enough water to cover them. Bring to a boil, and boil until tender, about 15 minutes. Drain the potatoes and add 4 Tablespoons vegan buttery spread, non-dairy milk, salt, and pepper to the pot. Use a potato masher or mixer to mash the potatoes. Set aside.
Check the liquid level on the lentil mixture - the liquid should be mostly absorbed, but there should still be some in the pot - the lentils should not be dried out.
Once the lentils are tender, add the ketchup, frozen peas, salt, pepper, paprika, Italian seasoning, and garlic powder. Cook over medium heat for about 5 minutes, adding a bit more water if needed.
Once everything is heated through, spread the lentil mixture into a 9x13 casserole dish. Top with the mashed potatoes, and use a spatula to spread them out evenly.
Dot the top of the casserole with a little more vegan buttery spread if desired.
Bake at 425 degrees F for about 15-20 minutes, just to let the potatoes get a little golden brown.
Remove from the oven and top with fresh parsley if desired. Enjoy!