Rib-Eye, New Potato and Portobello Kebabs with Rosemary

new potatoes, such as Red Bliss (about 3 ounces each)

large portobello mushroom cap (about 4 ounces)

14- to 16-ounce rib-eye steak

sea salt

freshly ground black pepper

extra-virgin olive oil

balsamic vinegar

chopped fresh rosemary

minced garlic

12-inch bamboo skewers, soaked in warm water for at least 30 minutes