Preheat the oven to 400°F.
Heat the olive oil and butter in a large skillet over medium heat. Once hot, add the mushrooms, onion, and garlic to the pan.
Cook until the onions are translucent and the mushrooms have released moisture, stirring frequently.
Add in the thyme leaves and flour.
Mix the flour into the butter and pour in the vegetable broth and almond milk.
Bring to a boil until it thickens.
Add in the frozen mixed vegetables, potatoes, and lentils. Season with salt and pepper.
Pour the vegetable mixture into a pie dish and set aside.
In a large bowl, combine the flour, salt, vegan butter, and ice water until a dough forms.
Place the dough between two sheets of parchment paper and roll it out to the width of your pie dish.
Carefully transfer the dough to the pie dish, crimping the edges down.
Make a few slits on top of the crust.
Transfer your vegetarian pot pie to the oven and bake for 45 minutes or until the top is golden brown.