Blanch the kale in boiling water for 1-2 minutes, rinse with cold water, squeeze dry and set aside. Cook the lentils in the same boiling water until tender, about 20 minutes, then drain. Boil the potatoes with one garlic clove in salted water until very tender, 18-20 minutes. Drain and mash with 2 tbsp olive oil, 2 tbsp non-dairy milk, salt and pepper. Chop the kale and fold it in. Preheat the oven to 375°F (190°C) and grease a 9x13 baking dish. Sauté the onion in the remaining oil for 3 minutes. Add the minced remaining garlic and herbs for 30 seconds. Add the mushrooms, cover for 2 minutes, season with pepper, and cover for 2 more minutes. Stir in the lentils, balsamic, tamari and tomato paste, scraping up browned bits, and season with salt. Add the stock and bring to a boil. Stir in a slurry of arrowroot and the remaining milk and simmer until thickened, about 3 minutes. Pour into the dish, spread the mash on top, drizzle with oil and sprinkle with pepper. Bake until bubbling and lightly browned, about 25 minutes.