Preheat oven to 375°F (190°C). Boil potatoes in salted water until fork-tender, about 15-20 minutes.
While potatoes cook, heat olive oil in a large skillet over medium heat. Sauté diced onion for 5 minutes until softened.
Add minced garlic and cook for 1 minute until fragrant. Stir in tomato paste if using.
Add rinsed lentils, vegetable stock, thyme, salt, and pepper. Bring to a boil, then reduce heat and simmer for 25-30 minutes until lentils are tender.
Drain potatoes and mash with vegan butter, salt, and pepper until smooth and creamy.
Stir frozen vegetables into the lentil mixture and cook for 2-3 minutes until heated through.
Transfer lentil mixture to a 9x13 inch baking dish. Spread mashed potatoes evenly over the top.
Bake for 15-20 minutes until bubbly around edges and golden on top.
Let cool for 5 minutes before serving.