Sear the sausages in the oil in a large skillet over medium-high heat until the internal temperature is 150 degrees F, about 5 minutes per side. Remove the sausages from the skillet and set aside. Return the skillet to medium-low heat and add more oil if the pan is dry. Add the onions and salt to taste and cook the onions until translucent and just beginning to brown, about 5 minutes.
Deglaze the skillet with the vinegar, scraping the bottom to get all the brown bits (fond), which equals flavor. Add the cabbage and apples and stir to combine. Let it cook down a bit, stirring occasionally, about 4 minutes. Season with salt and sugar and cook until the cabbage and apples start to soften, another 5 minutes. Return the sausages and any juices that have collected to the skillet. Cover and cook to heat through. Sprinkle in some sage and cook until the internal temperature of the sausages is 165 degrees F, about 5 minutes more. Garnish with additional sage. Adjust the salt if needed, then serve with crusty bread.