For the Gravy: Heat a large skillet over medium-high heat. Break off/crumble the sausage into little pieces with your fingers and add them to the pan. Cook until no longer pink. Add the onions and cook until translucent, 5-7 minutes. Add the bacon grease and flour, stirring to combine and cook until the mixture is golden brown, 5-7 minutes.
Add the milk gradually along with the seasonings, stirring constantly to prevent lumps. Simmer uncovered for 10 minutes or until the sauce is thickened, stirring occasionally. Add salt and pepper taste.
For the Buttermilk Biscuits:Preheat the oven to 425 degrees F. Line a baking sheet with parchment paper.
Combine the flours, baking powder, baking soda, sugar, and salt in a food processor and process briefly to mix. Add the butter and pulse until the mixture resembles coarse crumbs. Add the buttermilk all at once and pulse just until the ingredients are moistened. Don't over-pulse.
Scrape the sides and bottom of the food processor bowl and turn out the dough onto a floured work surface. The dough will be very soft. Sprinkle the dough with a little flour and gently knead the dough a few times. Pat the dough into a loose rectangle about 1 1/2 inches thick. Fold the rectangle like a letter, overlapping the short sides in the middle to make 3 layers. Pat the dough into a rectangle 1 inch thick. Use a 2 1/2 inch biscuit cutter dipped in flour to cut out the biscuits, pushing straight down and lifting up without twisting. Pack together the remaining scraps and cut out additional biscuits.
Place the biscuits on the baking sheet, spaced about 1 inch apart, and bake for about 15-18 minutes, or until the biscuits are firm to the touch and the tops and bottoms are golden brown. Let them rest for a few minutes before slicing and serving with the sausage gravy. Makes 16 biscuits.