Prep the Lentil Base:
Heat olive oil in a medium, heavy-bottomed pot over medium-high heat.
Add onions and sauté for 3-5 minutes, until tender and translucent.
Add garlic and sauté for 2-3 minutes longer.
Add lentils and stir to coat evenly with oil.
Add sprig of rosemary and sage leaves.
Pour vegetable stock over lentils and bring to a boil. Once boiling, reduce heat, cover, and allow to simmer for 30-45 minutes.
Add chopped potatoes to a pot and cover with cold water.
Bring to a boil over high heat and boil until softened, about 10-15 minutes.
Drain, return to pot, add milk, butter and salt. Using a potato masher or a mixer, mash the potatoes to your desired level of creaminess. Add more salt and a dash of black pepper to your taste.
When the lentils have about 15 minutes left, add the chopped carrots, celery, and mushrooms. Stir to combine, cover again, and allow to simmer.
Roast the Pie:
Preheat oven to 400 degrees F.
When the lentils have softened, remove sage leaves and rosemary stem from the pot, stir in 2-3 tablespoons of tomato paste, until you reach your desired consistency. Stir in chopped kale.
Transfer to a casserole dish.
Top with prepared mashed potatoes.
Place shepherd’s pie in the preheated oven. You will want a piece of foil or a pan underneath the dish in case the lentil mixture bubbles over the sides.
Bake for 15-20 minutes, until the lentils are bubbling and the potatoes have just begun to brown.
Remove from oven, allow to cool for 5-10 minutes, then enjoy!