Wash hands with soap and water.
Preheat oven to 400ºF.
Scrub sweet potatoes and onion with a clean vegetable brush under running water.
On a large baking sheet toss frozen Brussels sprouts, potatoes, and onion with oil, rosemary, salt, and pepper. Divide between 2 baking sheets so vegetables are in a single layer.
Cook the Brussels sprouts according to package instructions, stirring occasionally, until all the vegetables are tender and browned. This will take about 30-40 minutes
Transfer vegetables to a serving dish and toss with cranberries and pecans. Serve immediately.