Place the lentils in a pot. Cover with 1 inch cold water. Cover the pot, and bring to a boil. Reduce heat to low and simmer for 20 minutes. Drain the lentils and set aside.
Peel and rinse the potatoes. Cut the potatoes in half to speed up the cooking time. Put in a large pot, and fill the pot with water. Bring to a boil then reduce the heat and simmer until potatoes are tender. Drain the potatoes.
Mash the potatoes with a potato masher. Add the melted butter and warm plant milk, and mix until well incorporated. Season to taste with salt and pepper.
Heat a large pan on medium heat. Add the 2 Tbsp. oil. Once it shimmers, add the diced onions and sauté for 5 minutes.
Add the garlic, celery, carrot, and thyme, and cook 5 minutes more.
Sprinkle the flour, and stir to combine.
Add the broth, and stir to combine.
Add the lentils, and cook for 5 minutes.
Add the frozen peas and corn to the mixture. Season with salt and pepper. Cook for 3 minutes.
Add the mixture to a 11×7 oven-safe baking dish. Top with the mashed potatoes, starting around the edges to create a seal to prevent the mixture from bubbling up.
Bake the pie for about 25 minutes until the potatoes begin to brown. Allow the Shepherd’s Pie to cool 15 minutes before serving.
Notes
A key ingredient is to use French green lentils. They have a different flavor and are smaller than regular green lentils.