- Heat olive oil over medium heat in a large skillet. Add celery, carrots, mushrooms, and onion. Sauté for 10 minutes, until vegetables start to soften. Stir in garlic and cook 1 minute longer.
- Stir in thyme, sage, fennel, and pepper. Sprinkle flour over vegetables and stir to coat. Cook for 1 minute. Slowly stir in vegetable stock. Stir in tomato paste and soy sauce. Reduce heat to medium-low and bring mixture to a simmer. Simmer for 5 minutes to thicken.
- Add lentils and frozen peas to vegetables and stir to combine. Pour into a greased casserole dish or deep pie pan. Top with mashed potatoes and spread evenly. Mist potatoes with olive oil cooking spray or drizzle with olive oil. Bake for 30 minutes, turn oven to broil for the last 5 minutes to lightly brown potatoes.
- Serve hot garnished with additional fresh thyme.