How to Make It
Whisk together 1/2 cup of the cider and flour in a small bowl; set aside.
Heat oil in a large, deep skillet over medium-high. Add sausage, and cook, turning occasionally until all sides are browned, about 5 minutes. Transfer sausage to a plate (it will not be cooked through).
Add onion to skillet over medium-high, and cook, stirring constantly, 3 minutes. Add potatoes, salt, and pepper; cook, stirring often until onion is translucent, about 3 minutes. Add apple slices and remaining 1 cup cider, scraping bottom of skillet with a wooden spoon to release brown bits. Bring mixture to a simmer, and return sausage to skillet. Reduce heat to medium and cook, uncovered, until sausage is cooked through and liquid is reduced, about 15 minutes.
Using a slotted spoon, transfer sausage and vegetables to a serving platter.
Whisk reserved flour mixture, mustard, thyme, and sage into skillet, and cook, stirring constantly, until thickened, about 2 minutes. Drizzle over sausage and vegetables.