Preheat the oven to 425°F.
To make the biscuits, sift together the flour, baking powder, salt, and cream of tartar into a medium bowl. Stir in the sugar with a spoon. Using a pastry cutter or your fingers, blend in the shortening until the mixture is crumbly. Mix in the milk with a fork until the dough is soft but not sticky.
Place the dough on a floured board and knead it for a minute or so. Roll it out to a thickness of 1 1/4 inches and cut it into biscuits with a 2-inch round cutter or the rim of a small water glass. Arrange the biscuits 2 inches apart on a baking sheet.
Bake the biscuits for 18 to 20 miniutes, until lightly browned on top. Take the baking sheet out of the oven and set the biscuits aside.
To make the gravy, cook the sausage in a large skillet over medium-high heat until browned, about 8 minutes.
Sprinkle the flour over the meat and stir with a spoon until blended. Stir in the milk. Bring the mixture to a boil, stirring until thickened. Reduce the heat and simmer for 5 minutes. Season to taste with salt and pepper.
Cut biscuits in half horizontally and place them cut side up on plates. Spoon the gravy over the biscuits and serve immediately.