Whisk the flour, baking powder, and salt together in a large bowl or use a food processor. Add the cold butter and cut into the dry ingredients with a pastry cutter or pulse several times in the processor. Cut/pulse until coarse crumbs form. Add 1 cup (240ml) milk (reserve the rest for brushing on dough in step 4), and then stir/pulse until the dough comes together. Dough will be very shaggy and a little wet. If it’s too dry, fold in another Tablespoon of milk. If it’s too wet, fold in another Tablespoon of flour. With generously floured hands, shape biscuit dough into 8 or 9 1-inch-thick discs. The amount of biscuits you need (and their diameter) depends on the size and shape of the baking dish you’re using for the pot pie. Dough is sticky, so keep your hands floured. Biscuits don’t have to be perfect or neat. Place shaped biscuits on a lined plate or baking pan, cover tightly, and refrigerate until ready to use in step 4, or for up to 2 days.

Melt the butter in a large skillet, pot, or 11- or 12-inch oven-safe skillet over medium heat. Add the onion, carrots, celery, mushrooms, and garlic. Cook, stirring occasionally, for 5 minutes or until vegetables have softened and released some liquid. Stir in flour, salt, pepper, and thyme until flour has absorbed all the liquid. Stir in the broth and 1/2 cup (120ml) milk. Cook and simmer for 7–9 minutes or until thickened into a thick soup-like consistency. Stir in the mixed vegetables and parsley, and then remove from heat. Taste and add more salt, pepper, thyme, or parsley if desired. Cool for 5 minutes.

As it’s cooling, preheat oven to 400°F (204°C).

Pour filling into greased 2.5- or 3-quart baking dish, or use a 9-inch pie dish that’s 2 inches deep. Arrange cold biscuits on top, squeezing them in as necessary to fit. Brush the tops of the biscuits with remaining 2 Tablespoons (30ml) milk.

Bake for 25 minutes, and then, keeping the pot pie in the oven, turn oven up to 425°F (218°C) and bake for 5–6 more minutes, or until biscuits are golden brown on top.

Remove from the oven and allow to cool for 5 minutes before serving. Leftovers keep well in the refrigerator in an airtight container for up to 5 days. Reheat as desired.