1. Heat the oven to 425 degrees with a rack in the center position.
2. Toss together the first four ingredients on a rimmed baking sheet and season with salt and pepper. Spread the mixture evenly.
3. Use a fork to prick each sausage a few times, then tuck them among the other ingredients. Sprinkle on the rosemary.
4. Bake until the sausages are cooked and nicely browned, stirring halfway through, about 30 minutes.