Preheat oven to 400º F.
Add potatoes to boiling, salted water and cook until fork tender, about 15 minutes. Drain.
Mash potatoes, then add minced garlic and vegetable stock and mix until smooth. Add chopped thyme and salt & pepper to taste. Set aside.
Heat large skillet to medium high heat. Cook mushrooms for 5-7 minutes until liquid begins to evaporate.
Add carrots, onion, and celery and cook until vegetables begin to soften slightly, about 3-5 minutes. Add herbs, garlic, stock, and tomato paste and simmer for 7-10 minutes.
Fold in lentils to vegetable mixture, add salt & pepper to taste. Transfer lentil filling to an oven safe dish. Top with mashed potatoes.
Bake at 400º F for 15 minutes, or until top is browned slightly and filling is bubbly.