Preheat the oven to 425°F (220°C). Lightly grease a 9 x 13-inch baking dish or large rimmed sheet pan.
Add the cabbage, sliced onion, and red onion to the baking dish. Drizzle with olive oil and melted butter.
Sprinkle with garlic, smoked paprika, garlic powder, onion powder, thyme, salt, black pepper, and red pepper flakes. Toss until evenly coated.
Spread the vegetables into an even layer. Bake for 15 minutes.
Remove the pan from the oven, stir the cabbage, and add the sliced sausage.
Return to the oven and bake for 20–25 minutes, stirring once halfway through, until the sausage is browned and the cabbage is tender with caramelized edges.
For extra browning, broil for 2–3 minutes, watching closely.
Sprinkle with parsley and serve hot with cornbread, mashed potatoes, or crusty bread.
