Cook the Vegetables:
Cut potatoes and parsnips into evenly sized chunks. Place in a large stockpot, cover with cold water, salt generously, and bring to a boil. Simmer until fork-tender, about 20–30 minutes.
Make the Stew:
Meanwhile, heat olive oil in a large skillet over medium. Sauté onion and mushrooms until softened and golden, about 8 minutes.
Stir in garlic and tomato paste; cook 2–3 minutes until paste turns deep red.
Add red wine, 1 Tbsp rosemary, thyme sprigs, 1/2 tsp salt, and black pepper. Simmer until wine is reduced by half.
Thicken and Add Lentils:
In a small bowl, whisk vegetable broth, flour, and soy sauce. Pour into skillet and simmer 6–7 minutes until thickened.
Stir in cooked lentils and remove thyme stems. Reduce heat to low.
Mash the Roots:
Drain potatoes and parsnips, return to pot. Add milk, butter, remaining 1/2 tsp salt, and 2 tsp rosemary. Mash until smooth. Adjust seasoning to taste.
Serve:
Spoon mash onto plates, top with the lentil-mushroom stew. Garnish with fresh herbs if desired.