Preheat oven to 400 F. Place rack in middle position.
Line a sheet pan with parchment paper.
Toss cubed rutabaga with onion, olive oil, rosemary, salt and pepper and spread onto sheet pan in an even layer. Bake 20-30 minutes until tender.
Heat a medium-large frying pan over medium heat.
When hot, add butter and swirl to coat pan. Fry rutabaga until browned.
Serve with grilled or roasted meat and a side salad.