Cut fingerling or new potatoes in half, then cut onions into quarters. Place them in a heavy-duty zip-top bag. Drizzle with olive oil and toss gently to coat. Remove vegetables from bag, place in a baking pan, then sprinkle with chopped fresh rosemary, salt, and pepper. Bake 25 to 30 minutes at 450°, or until vegetables are tender, stirring every 10 minutes. Serve at room temperature.