Place potatoes in a large pot, cover with water, and bring to a boil. Cook until tender, about 20 minutes. Drain and return to pot.
Add vegan butter and almond milk to the potatoes. Mash until smooth. Season with salt and pepper to taste. Set aside.
Preheat oven to 400 degrees F.
In a large skillet, heat olive oil over medium heat. Add onion and garlic, sauté until softened, about 5 minutes.
Stir in lentils, vegetable broth, thyme, rosemary, and tomato paste. Bring to a boil, then reduce heat and simmer until lentils are tender, about 25 to 30 minutes.
Add frozen mixed vegetables and vegan Worcestershire sauce to the lentil mixture. Cook for another 5 minutes.
Transfer the lentil mixture to a baking dish. Spread the mashed potatoes evenly on top.
Bake in the preheated oven for 20 minutes, or until the potatoes are slightly golden.
Allow to cool for a few minutes before serving.