Step 1

Preheat oven to 475 degrees, with a rimmed baking sheet on center rack. Toss squash with 1 tablespoon oil; season with salt and pepper. Add 2 tablespoons oil to baking sheet, then cabbage, squash, and sausages in a single layer. Drizzle cabbage with more oil; season generously with salt and pepper. Roast 15 minutes.

Step 2

Toss onion and sage with 1 tablespoon oil; season with salt and pepper. Flip sausages and vegetables; add onion mixture to sheet. Continue to roast until vegetables are tender and sausages are cooked, 20 minutes.

Step 3

Stir together vinegar, mustard, remaining 3 tablespoons oil, 1/4 teaspoon salt, and 1/8 teaspoon pepper. Serve sausages and vegetables with a drizzle of vinaigrette.