Adjust oven rack to middle position and preheat oven to 450 degrees. Line a baking sheet with parchment paper.
In a food processor, pulse flour, sugar, baking powder, baking soda, salt, butter, and shortening in until mixture resembles coarse meal, about 15 pulses. Transfer to large bowl. Stir in buttermilk until combined.
On a lightly floured counter, turn out dough. Knead until smooth, about 8 to 10 times. Pat dough into 9-inch circle, about 3/4 inch thick.
Using 3-inch biscuit cutter dipped in flour, cut rounds of dough (do not twist cutter) and arrange on prepared baking sheet. Gather remaining dough, pat into 3/4-inch-thick circle, and cut out remaining biscuits. (You should have 8 biscuits total.)
Bake until biscuits begin to rise, about 5 minutes, then rotate pan and reduce oven temperature to 400 degrees. Continue baking until golden brown, 12 to 15 minutes longer, then transfer to wire rack and let cool. (Biscuits can be stored in zipper-lock bag for up to 2 days.)
To make the gravy:
In a small bowl, Combine flour, fennel, sage, and 1 1/2 teaspoons pepper. In a large skillet over medium-high heat, cook sausage until no longer pink, about 8 minutes. Break up any clumps with a wooden spoon and do not drain.
Sprinkle flour mixture over sausage and cook, stirring constantly, until flour has been absorbed, about 1 minute.
Slowly stir in milk and simmer until gravy has thickened, about 5 to 7 minutes. Season with salt to taste (I like 1 teaspoon). Serve over split biscuits.
Baking powder and baking soda: My homemade biscuit recipe calls for both baking powder and baking soda to yield the fluffiest, loftiest results.
Butter and vegetable shortening: A two-to-one ratio of flavorful butter and flaky shortening leads to the best homemade biscuits. Cut both into 1/2-inch cubes, then chill before proceeding with the recipe. Cold fat is key.
Buttermilk: To make your own buttermilk, in a small bowl, whisk 1 tablespoon lemon juice into 1 ¼ cups of milk. Let stand until slightly thickened, about 10 minutes.
Yield: This recipe makes 8 biscuits and 5 cups gravy, enough for 8 generous servings (with a heaping 1/2 cup gravy each).
Storage: Store extra biscuits in a zip-top bag for up to 2 days. Transfer leftover gravy to an airtight container and refrigerate for up to 4 days. Reheat the biscuits in the microwave or oven, and reheat the gravy in a saucepan on your stovetop, thinning with milk if the gravy appears too thick.
Freezer: While you can freeze raw biscuits successfully, the gravy is best made to order, the day you need it.