Heat your oil in a large stock pot over medium heat.
Add your onion, celery and mushrooms, and cook for about 10 minutes, until the mushrooms are brown and the onions are nice and soft.
Add your garlic and carrots, cook for 2 minutes, then add your vegetable stock.
Toss in your rosemary, cover, and bring to a boil. Lower your heat and continue on a low simmer for 45 minutes. Remove your rosemary sprig and discard. Add your lentils, and give it a nice stir.
Preheat your oven to 425°F. While your vegetables are simmering, boil your potatoes until soft enough to mash. Drain them well, and add your butter to the hot pot. Melt your butter, then add your potatoes back to the pan, and also the milk. Mash well to your desired texture, and salt and pepper to taste.
In a 9 x 13 baking dish, layer your vegetables. Spread your frozen peas evenly over the veggies, then spread your potatoes over the evenly over the top. Bake for 30 minutes, until bubbly and the top starts to brown. Let sit for ten minutes, then enjoy!