Preheat oven to 425°. Spread a large rimmed baking sheet with 2 tbsp. oil. Separate onion into rings of 2 to 3 layers. Set onion, sausages, brussels sprouts, and apples in pan. Liberally salt and pepper everything. Drizzle with remaining 2 tbsp. oil, toss to coat, and arrange evenly.
Bake until vegetables are tender and meat is cooked through, 25 to 30 minutes. Drizzle vinegar on top and sprinkle with parsley.