Melt the butter in a large skillet over medium to medium-high heat. Add the mushrooms, carrots celery and herbs and saute until vegetables are just tender, about 6-7 minutes. Sprinkle the flour over the vegetables and stir until well combined. Add the broth a little at a time, stirring constantly. Bring to a low boil and cook for about 1 minute until the sauce thickens. Taste for seasoning and add a little salt and pepper if necessary - but don't go overboard as the biscuits have salt and pepper in them as well.
Remove from heat and stir in the frozen peas and pearl onions. Set aside.
In a medium bowl combine the flour, baking powder, baking soda, salt, pepper and scallions. Add the butter and cut the butter into the flour using a pastry cutter or 2 knives until it resembles coarse meal - it's ok for a few larger pieces of butter to be visible. Add the buttermilk and stir until just combined.
Flour a work surface and turn the dough out onto it. Knead the dough once or twice, then pat it into a rectangle, about 3/4" thick. Use 2-3" biscuit cutters to form the biscuit. Re-assemble the scraps together, pat to 3/4" and cut out more biscuits.
Spray a casserole dish with vegetable spray and transfer the turkey mixture to the dish. Arrange the biscuits on top of the filling. Brush the biscuits with the cream and sprinkle with a little additional pepper.
Bake for 25-30 minutes until biscuits are golden brown and filling is bubbling. Serve.
You may end up with extra biscuits - which can be cooked at the same time the pot pie is baking -- but the biscuits take about 5 minutes less time than the pot pie, so be vigilant and remove them accordingly.