Cook green lentils as per package instructions (can also use canned).
Preheat oven to 400 degrees F. Grease a large casserole dish with vegan butter or oil and set aside.
Sauté onion and garlic in a large pan over medium heat until translucent, about 5 minutes.
Meanwhile, add diced potatoes to a large pot of water. Bring to a boil, and cook until potatoes are fork-tender, approximately 15 minutes.
Add carrots, celery, mushrooms, cooked lentils, salt, pepper, thyme, rosemary and soy sauce to the pan. Sauté until combined, approximately 3 minutes.
Whisk together vegetable stock and flour. Add to pan with filling.
Sauté filling for 5-10 minutes, until you notice it starts to thicken and carrots and celery are fork-tender.
Meanwhile, drain cooked mashed potatoes. Place back in the pot, and mash in vegan butter, unsweetened plant-based milk, salt, and garlic powder.
Spoon lentil mixture into the casserole dish, spreading evenly. Spread potato mixture on top. Feel free to drag a fork throughout the potatoes to create divets, which will get crispy in the oven!
Bake for 25 minutes, until lentil mixture is bubbling slightly up the sides of the dish and edges of the potato are golden brown. Serve and enjoy!