For the Potatoes

1. Add potatoes to a large pot and cover with water. Bring to a boil over medium high heat. Cook for 15 more minutes until potatoes break apart when pierced with fork.
2. Drain the potatoes. In a large mixing bowl, add potatoes, butter, sour cream, salt and milk and mash together until completely combined. Cover and keep warm.

For the Filling

1. Preheat oven to 400˚F. Prepare a 9X13 casserole dish or use an oven safe sauté pan to make the filling.
2. In a small pot, bring lentils, Italian seasoning, and 2 cups of the vegetable broth to a simmer. Cook partially covered until tender and most water is absorbed, about 20 minutes. Set aside in a bowl.
3. While the potatoes are boiling, heat olive oil in a large skillet over medium high heat.
4. Add the mushrooms and cook 8-10 minutes, stirring frequently, until somewhat softened and they've started to release their juices
5. Add onion, carrots, celery, and leeks and sauté until the onion starts to turn translucent, about 4 minutes.
6. Add the garlic and sauté for another minute or until fragrant.
7. Add the lentil mixture, stirring frequently for about 4-5 minutes.
8. Add the tomato paste and flour to the skillet and sauté for 2 minutes.
9. Pour in red wine and cook down for about 2 minutes while scraping any browned bits off the bottom of the pan.
10. Pour in the remaining vegetable broth. Bring to a boil and then reduce heat to a simmer, cooking down for another 10 minutes.
11. Add the peas and lemon juice, stirring to combine. Remove from heat. Add salt and pepper to taste.
12. If using a casserole dish, pour lentil mushroom mixture into the dish and press into a smooth layer and add the mashed potatoes on top. If using an oven safe skillet, leave the filling in the pan and simply add mashed potatoes on top! Seal in the sides to avoid too much bubble up from the filling below.
13. Cook for 25-30 minutes until potatoes have just started to brown and any liquid on the sides is bubbling.
14. Cool on a wire wrack for 10 minutes. Garnish with freshly ground black pepper and chives. Serve immediately.