Cut the potatoes and parsnips into equal sized chunks. Place them in a large pot, cover with cold water, and generously salt. Bring to a boil and simmer until the vegetables are fork tender, about 20 to 30 minutes.
Meanwhile, heat oil in a large skillet over medium. Add onion and mushrooms; cook until onion softens and mushrooms are golden, about 8 minutes. Add garlic and tomato paste; cook until tomato paste turns brick red, about 2 to 3 minutes.Add wine, 1 Tbsp. rosemary, thyme, ½ tsp. salt, and black pepper. Bring mixture to a simmer, and cook until the wine reduces by half.Whisk together broth, flour, and soy sauce in a bowl, and pour into pan. Simmer until mixture thickens, about 6 to 7 minutes. Stir in lentils, and remove thyme stems from pan. Reduce heat to low.
Drain potatoes and parsnips and place back in hot pot. Add milk, butter, remaining ½ tsp salt, and 2 tsp. rosemary. Use a potato masher to thoroughly mash. Taste and adjust seasonings as needed.Divide potato-parsnip mixture evenly onto 4 plates. Spoon lentil and mushroom mixture overtop. Garnish with additional herbs, if desired.
