Preheat the oven to 425F. Cover a baking sheet with a piece of parchment paper.
Make the biscuits: sift the flour, baking powder, and salt together into a large bowl. Using two knives or a pastry blender, cut in the butter until the texture rangers from cornmeal to the size of small peas. (These two steps can be done quickly in a food processor.) Using a fork, gradually mix in the yogurt, then the milk, until the dough is quite soft and somewhat sticky. Work quickly without overworking the dough. Turn it out onto a lightly floured surface and knead it briefly, just long enough to get it to hold together. Roll it out so it is about 1 1/4 inches thick, and cut it into 2 inch rounds. (You will have 10 biscuits.) Arrange the rounds on the prepared baking sheet, and bake in the center of the oven until they are deep golden, puffed and cooked through, 18 to 20 minutes.
While the biscuits are baking, prepare the sausages: place all the sausage ingredients in a large bowl and mix thoroughly, using your fingers. To test the seasoning, fry a tiny patty and taste; adjust if necessary.
Place the patties in a large heavy or nonstick skillet over medium-high heat, and cook until they are crisp and brown on the outside and cooked through inside, about 8 minutes per side. Transfer the sausage patties to a warmed platter and keep warm.
Make the gravy: Add enough of the butter to the skillet so the bottom is covered with 1/8 inch of fat. Stir, scraping up the browned bits from the bottom of the skillet. Reduce the heat to medium and add the flour slowly, stirring until it has absorbed the butter. Cook, stirring constantly, until it turns golden brown, at least 2 minutes. Then slowly pour in the milk, stirring constantly, and cook until the gravy thickens to the consistency of very heavy cream. Season to taste with salt and a generous amount of black pepper.
To serve, split two biscuits in half. Place the bottom halves on a warm plate, top them with sausage patties, and pour a generous amount of gravy over the sausage. Mince the fresh sage and sprinkle some over the sausage and gravy. Set the biscuit tops at an angle, partially covering the sausages and serve immediately.
4 servings (with extra biscuits).