Heat up the olive oil in a large saucepan or stockpot over medium heat.
Saute the onion in the olive oil for 5 minutes until soft.
Add carrots, mushrooms, garlic, thyme, and rosemary, and cook for another 5 minutes on medium-high heat.
Add the lentils and vegetable stock and bring to a simmer. Turn the heat down to low-medium and simmer for 20-25 minutes. In the meantime, make the mash (see below).
Once the lentils are done and have absorbed most of the broth, add the peas, soy sauce, tomato puree / paste, smoked paprika, mustard, brown sugar and black pepper. Stir to combine and simmer for 5 more minutes.
Mash Topping
While the lentils are cooking, start making the mashed potato by submerging the cut potatoes in water in a second saucepan and bring to a boil.
Simmer the potatoes for about 15 minutes until fork tender.
Drain the potatoes. Put them back in the pan together with vegan butter, salt, and non-dairy milk. Mash until completely smooth.
Bake
Preheat the oven to 200℃ / 390℉.
Spread out the lentil filling evenly in a large baking dish.
Top the lentil filling evenly with the mashed potato, then smooth the surface with a silicone spatula or the back of a spoon.Top Tip: Put some mash along the edges of the dish first before working your way to the center. This way, you can keep your filling from subsiding if it gives way easily, and ensure even distribution.
Optionally, run a fork across the surface of the potato to draw a wave, spiral, or another pattern.
Cook in the oven for 15 - 20 minutes at 200℃ / 390℉. For a more golden brown crust, optionally place under the grill/broiler for an extra 5 - 10 minutes.
Notes
Dish sizeWe used a pyrex dish 35 x 23 cm in size with a 2.8L or 3 quart volume.Store leftovers in the fridge for up to 3 days in a covered dish or container.You can freeze this lentil shepherd's pie before or after baking. See the post above for details.