How to Make It
Preheat the oven to 400° and lightly grease a baking sheet. In a large bowl, combine the flour, baking powder, sugar and salt. Using a pastry blender or 2 knives, cut the butter into the flour until it resembles small peas. In a small bowl, combine the buttermilk, onion, egg and sun-dried tomatoes. Stir the buttermilk mixture into the flour just until incorporated. Transfer the dough to a well-floured surface and knead 2 or 3 times.
Pat or roll the dough into a 7-inch square, about 3/4 inch thick. Cut the dough into 4 squares and place on the prepared baking sheet. Reduce the oven temperature to 375° and bake on the center rack of the oven for about 25 minutes, or until the biscuits are golden on the bottom and cooked through.
In a medium bowl, combine the sausage with the onion, 1 tablespoon of the parsley, the sage and a generous pinch each of salt and pepper. Scoop walnut-size balls of the sausage mixture onto a large plate. In a large skillet, heat the oil. Add the sausage balls and cook over moderately high heat until browned all over, about 5 minutes. Drain the sausage balls in a strainer set over a bowl.
Pour 1/4 cup of the fat back into the skillet. Add the mushrooms and leek and cook over moderately high heat, stirring occasionally, until softened, about 5 minutes. Stir in the flour and cook, stirring, until lightly golden, about 2 minutes. Gradually stir in the stock and cook over moderately low heat, stirring occasionally, until thickened, about 5 minutes. Return the sausage balls to the skillet and heat through. Season with salt and a generous pinch of pepper and stir in the remaining 1 tablespoon of parsley.
Split the biscuits in half and set the bottoms on 4 dinner plates. Top with the sausage gravy and biscuit tops.