Preheat the oven to 400 degrees F
Bring a pot of water to boil and add the dry lentils. Cook for about 20 minutes until soft, but not mushy.
Meanwhile, mince the garlic, dice the onion, carrots, celeriac, and mushrooms.
Heat olive oil in a large oven-proof skillet over medium heat until it shimmers then sauté the onion until translucent, about 5 minutes.
Add the carrots, celeriac, and garlic and continue to cook another 5 minutes.
Add the mushrooms, salt, thyme, paprika, and pepper and cook another 5 minutes.
Add the tomato paste and flour and stir for 1 minute until the vegetables are coated.
Add the vegetable broth and use a wooden spoon to scrape the brown bits of flour and tomato paste from the bottom of the skillet. Bring to a simmer and then stir in the cooked lentils.
Evenly distribute the mashed potatoes over the top of the vegetables and use the back of a spoon to spread it out until the potatoes entirely cover the rest of the mixture.
Bake the Shepherd's pie for 15 minutes until hot then turn on the broiler and cook another 1-2 minutes until the potatoes are nice and browned on top.