Cover diced potatoes with water in a large pot. Bring to a boil over high heat. Boil until softened, about 10-15 minutes depending on the size of your potato pieces.
While the potatoes are boiling, start cooking the lentil filling. When potatoes are fork tender, drain the water and return them to the pot. Add melted butter, soy milk, salt and pepper. Mash until softened to mashed potato consistency.
Lentil Filling
Preheat the oven to 350 degrees F. Make the corn starch slurry by stirring together 2 tablespoons corn starch and 2 tablespoons water in a small bowl. Set aside.
Combine the dry lentils and water in a medium sauce pot with a pinch of salt. Bring to a boil over high heat. Reduce to a simmer over medium-low and simmer for 15 minutes, or until lentils are tender. Once cooked, drain off excess liquid and set lentils aside.
Heat a 12 inch skillet over medium-high heat. Once hot, drizzle in olive oil. Add the mushrooms, onion, carrots, celery and chopped walnuts. Sprinkle with salt. Sauté until softened, stirring occasionally for even cooking, about 10-12 minutes.
Add in the minced garlic, wine, tomato paste, soy sauce, nutritional yeast, rosemary, thyme. Cook for another 1-2 minutes, until the garlic is aromatic.
Add in the vegetable broth, corn starch slurry and lentils. Stir and sit over medium-low. At this point the potatoes are usually ready to mash.
Layer the lentil mushroom filling into a lightly oiled 2 quart casserole dish or keep in the cast iron pan. Top with a layer of mashed potatoes.
Cover with foil and bake for 20 minutes at 350 degrees F. Remove the foil and bake another 15 minutes, until the potatoes are golden. Remove from the oven and let rest for 5 minutes before serving. Enjoy!
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Notes
Potatoes: Russet or Yukon gold potatoes work. I usually leave the skins on but you can peel them if you prefer.
Lentils: Use green or brown lentils. ¾ cup dry lentils = 2 and ½ cups cooked. Or about two cans of lentils.
Mushrooms: I roughly chop them so that they are noticeable in the filling, but if you or someone you're serving doesn't like mushrooms you can chop them very finely or reduce to just 8 ounces. I use a mix of cremini mushrooms and baby bella mushrooms. White mushrooms also work.
Wine: I use red wine, usually cabernet or pinot noir. The alcohol cooks off during the cooking process and leaves a rich flavor.
Storage and reheating: Cover and keep refrigerate for 3-4 days for best quality. Reheat single serving in a microwave. You can also cover the casserole with foil and reheat it in the oven at 325 degrees. It will take 15-20 minutes for the filling to warm through.