Preheat over to 425 degrees F.
Peel and slice any large potatoes in half. Place in a large pot and fill with water until they’re just covered. Bring to a low boil on medium high heat. Cover and cook for 20-30 minutes or until they slide off a knife very easily.
Once cooked, drain and then return to pot. Mash with a potato masher and add vegan butter. Set aside.
In a large saucepan, heat olive oil. Once heated, add onion and garlic, until slightly browned (about 5 minutes).
Once browned, add frozen veggies, along with 1/4 cup of water or veggie broth to saucepan. Heat until veggies are unfrozen. Add cooked lentils and thyme to saucepan for for 1-2minutes, until flavours meld. Mash slightly with a potato masher to thicken.
Transfer veggie and lentil mixture to lightly-greased 2-quart baking dish. Top with potato mash.
Bake in oven for 15 minutes. Broil on high for an addition 5 minutes, or until potatoes are browned.
Let cool before serving.