Preheat the oven to 350°F.
Season the chicken with salt and pepper and arrange on a baking sheet. Roast until cooked through, about 22-28 minutes.
Let the chicken breast cool slightly, then dice.
Increase the oven to 400°F.
Arrange the oyster mushrooms on a baking sheet, toss with the canola oil and a pinch of salt and pepper. Roast for 8 minutes, until softened.
In a large saucepan, melt the butter. Stir in the flour and cook for 7-10 minutes, stirring constantly, until the flour no longer smells raw.
Stir in the chicken stock, thyme and a pinch of salt and pepper, and bring to a simmer. Add the milk, roasted mushrooms, and carrots, return to a bare simmer, sir in the peas, and remove from heat. Add in the parsley. Adjust the seasoning.
Divide the chicken between bowls, top with pot pie base and a biscuit. Sprinkle with parmesan cheese.