Preheat oven to 400 F. Drain beets. Discard liquid or save for another use.
In large bowl, toss sweet potatoes, olive oil, salt, pepper and onion until coated.
Place silicone mat on baking sheet or line with aluminum foil.
Pour contents of bowl onto baking sheet in single layer. Remove rosemary from stems; sprinkle over vegetables. Bake 30 minutes until potatoes are almost tender.
Add beets to baking sheet. Return to oven 10-15 minutes, or until potatoes are fork-tender and beginning to brown. Garnish with additional rosemary, if desired.