Food processor method: Add the flour, baking powder, and salt into the food processor and pulse a few times to mix. Scatter the vegan butter cubes over the flour and pulse several more times until the mixture is a crumbly sandy texture. Pour over the plant-based milk and pulse a few more times until you reach a soft sticky dough (don't over mix it). Chill the dough in the fridge until you are ready to use it.
Bowl method: Add the flour, baking powder, and salt into a large bowl and whisk to combine. Scatter the vegan butter cubes over the flour and use a fork or pastry cutter to cut the butter into the flour until you reach a crumbly sandy texture. Drizzle the plant-based milk across the flour mixture and use the fork to combine, until you reach a soft sticky dough (don't over mix it). Chill the dough in the fridge until you are ready to use it.
Preheat your oven to 400F (200C). Lightly grease a 13" x 9" casserole dish (4.5 quarts)
Melt 2 tablespoons of vegan butter in a large pot over medium-high heat. Add the onion, carrots, celery, mushrooms, and garlic and sauté until the veggies begin to soften, 5 - 10 minutes. Sprinkle over the flour and stir to coat the veggies. Stir in the vegetable broth and add the potato, nutritional yeast, thyme, salt, and pepper. Bring to a simmer and cook 7 - 10 minutes until thickened. Stir in the plant-based milk and peas and remove from heat.
Carefully pour the veggie filling into the prepared baking dish. Take the chilled biscuit dough and divide it into eight. Using your hands, take one piece of dough and gently form it into a rough biscuit shape. Don't worry if it's messy. Place on top of the pot pie filling and repeat to make 8 biscuits. Lightly brush the biscuits with a bit of plant-based milk.
Place the prepared pot pie in the oven and bake for 25 minutes. After 25 minutes, turn the temperature up to 425F (220C), and continue to bake for another 5 - 8 minutes until the biscuits are golden. Remove from the oven and allow to cool for 5 - 10 minutes before serving.