Start the Lentil Stew: Heat olive oil in a large pot over medium heat. Add diced onion and sauté for 3–4 minutes until softened. Add garlic, carrots, and celery. Cook another 5–6 minutes until slightly softened.
Build Flavor: Stir in tomato paste, Dijon mustard, and soy sauce. Cook for 1–2 minutes to deepen flavor.
Simmer the Stew: Add lentils, vegetable broth (start with 4 cups), thyme, rosemary, bay leaf, and black pepper. Bring to a boil, then reduce heat and simmer uncovered for 25–30 minutes, until lentils are tender.
Cook the Potatoes: While the stew simmers, add chopped potatoes to a separate pot and cover with cold water. Bring to a boil and cook for 15–20 minutes, until fork-tender. Drain and keep warm.
Finish the Mashed Potatoes: Mash the potatoes while still hot. Add vegan butter and mix until melted. Slowly add warm soy milk (start with 1/4 cup) until creamy. Season generously with salt.
Thicken the Stew: Remove and discard the bay leaf. Stir in the cornstarch slurry and simmer for 2–3 minutes. Remove about 1 cup of the stew, blend until smooth, then stir back in. Taste and adjust seasoning.
Serve: Spoon mashed potatoes into bowls. Ladle thick lentil stew over top. Finish with black pepper or fresh herbs if desired.