Heat the butter and oil in a large skillet over medium high heat. Add the onions to the skillet and brown lightly. Add the mushrooms and herbs cook until just softened. Add the parsnips and potato. Cook for 1 minute. Stir in the flour so that all of the vegetables are coated. Cook for 1 minute. Add the stock, then the cream, salt, and pepper. Allow the sauce to thicken 5-10 minutes. Remove from heat and cool to room temperature before filling the pie.