Preheat the oven to 450°F and prepare a lightly floured work surface and an ungreased baking sheet.
In a bowl, whisk together the flour, sugar (if using), and salt.
Cut in the shortening and cold butter until the mixture looks crumbly, working quickly so the fat stays cold.
Add the buttermilk and gently stir just until the dough comes together. Add a little more buttermilk if needed.
Turn the dough onto a floured surface, fold it over itself a few times, and shape into a ¾-inch thick round.
Cut biscuits using a floured cutter, pressing straight down. Place them on the baking sheet.
Bake for 15–18 minutes, rotating the pan halfway, until lightly golden. Brush with melted butter if desired.
Make the Sausage Gravy
Brown the sausage in a saucepan over medium-high heat, breaking it into pieces. Add the onion and cook until soft.
Remove the sausage, leaving about 3 tablespoons of drippings in the pan, adding butter if needed.
Return sausage to the pan, sprinkle with flour, and cook until lightly golden and bubbling.
Stir in seasonings, then slowly add milk. Cook, stirring occasionally, until thick and creamy.
Serve the gravy generously spooned over split biscuits.
