To prepare the pot pie filling, steam the potatoes, carrots, parsnips, celery and mushrooms until al dente.  

Heat a large nonstick fry pan over medium-high heat. Add 1 1/2 teaspoons of the olive oil and swirl the pan to coat. Add the steamed vegetables along with the peas and pearl onions and cook, stirring occasionally, for 5 minutes. Transfer the vegetables to a large bowl. 

In the same pan over medium-high heat, warm the remaining 1 1/2 tablespoons olive oil. Stir in the flour with a whisk and cook, whisking constantly, for 1 minute. Gradually add the milk and broth, whisking constantly. Reduce the heat to medium and cook, whisking constantly, until the sauce is thick and bubbly, about 3 minutes. Add the vegetables to the sauce and cook until the sauce has a rich, semi-thick consistency. Stir in the salt, pepper and thyme.

To prepare the biscuits, preheat the oven to 450°F.

In a bowl or in a food processor, combine the flour, baking powder, baking soda, salt and pepper. Cut the butter into the flour until the mixture resembles coarse meal. If using a food processor, pulse a few times until this consistency is achieved. Add the buttermilk and mix just until combined. If the dough appears dry, add a bit more buttermilk. It should be very wet.

Turn the dough out onto a floured surface. Very gently pat the dough out (do NOT roll with a rolling pin) until it's about 1/2 inch thick. Fold the dough about 5 times, then gently press the dough down until it's about 1 inch thick. Using a round cutter, cut the dough into rounds. You can gently knead the scraps together and make a few more biscuits, but they will not be anywhere near as good as the first ones. 

Place the biscuits on a cookie sheet. If you like biscuits with soft sides, place them so they touch each other. If you like crusty sides, place them about 1 inch apart; they will not rise as high as biscuits that are touching.

Bake until the biscuits are a beautiful light golden brown on top and bottom, 10 to 12 minutes. 

To assemble the pot pie, make sure the filling is hot when the biscuits come out of the oven. Transfer the filling to a shallow dish, place the biscuits on top and serve.