1 small yellow onion, thinly sliced (about 2 scant cups)
1/2 small head green cabbage, cored, halved crosswise and sliced 1/4-inch thick (about 6 cups)
1/4 teaspoon crushed red pepper
2 cloves garlic, minced
3 tablespoons apple cider vinegar
1 tablespoon unsalted butter
1/2 small sweet-tart apple such as Gala or McIntosh, cored and sliced 1/8-inch thick (about 3/4 cup)
1 large scallion, thinly sliced (about 1/4 cup)
Hot sauce, for serving
Over medium-high heat, preheat a sizable cast-iron skillet or high-sided saute pan. To uniformly coat the pan, add the oil and swirl the pan. Sausage slices should be added in a single layer and cooked for 2 minutes, or until browned on the first side. About 2 minutes after flipping, grill the other side until it is browned. Slices of sausage should be transferred to a bowl with a slotted spoon and left aside.
Over medium-high heat, add the onion, a splash of water, and a generous amount of salt. While cooking, use a wooden spoon or heatproof rubber spatula to scrape up any browned bits from the pan's bottom. For 5 to 7 minutes, or until the onion is beginning to get soft and lightly browned in areas, cook, stirring periodically. Cook the cabbage for 6 to 8 minutes, stirring periodically, until it is crisp-tender. Add the crushed red pepper and another pinch of salt. Add another splash of water if the pan ever feels dry.
Cook the garlic and cider vinegar together for approximately a minute, stirring often, until the vinegar has mostly evaporated. Returning the sausage to the pan when the butter has melted and adding the apple, stir occasionally for 3 to 4 minutes, or until the apple slices are just starting to get tender.
Add the scallion and serve right now with spicy sauce on the side.