In a bowl, combine 1 1/2 cups of the all-purpose flour, the whole wheat pastry flour, baking powder, salt and baking soda. Using a pastry cutter or two knives, cut the butter into the flour mixture until it resembles coarse meal. Combine the buttermilk and 1 tablespoon of the oil. Pour the buttermilk mixture into the flour mixture and gently mix until just combined.
Turn the dough out onto a lightly floured surface. Knead lightly using the heel of your hand to compress and push the dough away from you, and then fold it back over itself. Give the dough a small turn and repeat eight or so times. (It's not yeast bread; you want to just barely activate the gluten, not overwork it.) Using a lightly floured rolling pin, roll the dough out 1/2 inch thick. Cut out rounds of dough with a 1 1/2-inch round cutter dipped in flour; press the cutter straight down without twisting so the biscuits will rise evenly when baked.
Meanwhile, heat the remaining 1 tablespoon oil in a large skillet over medium-high heat. Add the sausage and sprinkle with salt and pepper. Using a wooden spoon, break up the meat into small pieces. Cook until lightly browned, 3 to 5 minutes. Pour off any excess rendered fat. Add the onions and cook until clear and translucent, 3 to 5 minutes. Add the remaining 1/4 cup all-purpose flour and stir to combine and coat. Add the milk and season with salt and pepper. Bring to a boil and then reduce the heat to simmer. Let cook, stirring occasionally, until thick, about 5 minutes. Taste and adjust for seasoning with salt and pepper.
Meanwhile, place the biscuits on an ungreased baking sheet about 1 inch apart. Bake until golden brown, 8 to 10 minutes. Transfer to a rack to cool just slightly. Split and serve hot with the sausage gravy.