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