Melt butter in heavy large saucepan over medium heat. Add beets and onion. Cover; cook until onion is soft, about 8 minutes. Mix in rice. Add broth and vinegar. Increase heat; bring to boil. Reduce heat to medium-low. Simmer uncovered until rice and beets are just tender and risotto is creamy, stirring occasionally, about 15 minutes. Season with salt and pepper. Spoon into shallow bowls. Sprinkle with greens and cheese.