1. Make the polenta biscuits: Preheat the oven to 350°. In a large mixing bowl, whisk together the flour, cornmeal, baking powder, sugar, salt and baking soda. In a small bowl, whisk the buttermilk with the melted butter, then pour over the dry ingredients. Mix with a rubber spatula until the dough comes together in a ball.
2. Using a measuring cup, form six ½-cup clumps of dough, dropping each onto a baking sheet lined with parchment paper or a silicone mat. Bake for 25 minutes or until lightly golden on the bottom.
3. While the biscuits bake, make the sausage gravy: In a large skillet over medium heat, cook the sausage, breaking it up with a wooden spoon as it cooks, until browned, about 10 minutes. Remove with a slotted spoon and reserve.
4. Add the flour to the fat in the pan and cook, stirring, for 2 minutes. Add the milk, black pepper, red pepper flakes, sage and bay leaf and increase the heat to medium-high. Bring the gravy to a simmer and cook until it is thick enough to coat the back of a spoon, about 2 minutes. Remove from the heat and discard the sage and bay leaf. Return the sausage to the pan and stir in the vinegar and salt.
5. Divide the biscuits (halved if desired) among 6 plates. Top with the gravy and serve.