Soup is a great way to use up leftover greens, especially collards, or the leafy tops of beets or turnips, which too often get tossed in the compost pile. In this suave, satisfying soup, braising beet tops and collard greens with fennel and onion mellows the greens' hearty flavor and brings out a surprising smoothness (heightened by the cream stirred in at the end). Adding the spinach just before blending helps keep the color bright.