In a large pot of water, boil the potatoes until fork tender, about 20 minutes.
While the potatoes are cooking, heat the olive oil in a large skillet over medium heat. Add the garlic and onion and sauté until the onions are translucent, about 5 minutes. Add the lentils, soy sauce, tomato paste, Worcestershire sauce, parsley, salt, and pepper and stir to combine. Cook over medium low heat for a few minutes and then add the frozen mixed vegetables. Add the water, stir, and let the filling simmer over low heat for another 10 minutes.
Meanwhile, drain the potatoes and add them back to the pot with the butter, milk, and salt. Mash the potatoes with a potato masher or fork until you achieve the desired mashed potato texture. Set aside.
Preheat the oven to 400°F. Cut the tops of the peppers off and remove the seeds from the inside. Place them in a baking dish. Fill each pepper ¾ of the way up with the filling mixture and then top with the mashed potatoes.
Cover the baking dish with foil and bake in the oven for 35-40 minutes, remove the foil and broil the peppers for an additional 5-7 minutes, watching carefully so they don’t burn. Remove from the oven and enjoy!