Run drained beets through juicer. Discard pulp, or reserve for another use. Pour juice into small, low-sided skillet or saute pan, whisk over medium heat until bubbling around edges. Add rosemary sprigs and salt. Cook, whisking, to desired consistency. The beets are self-thickening, but don't cook too long. Whisk in red wine, remove from heat. Remove rosemary sprigs. If using, whisk in butter until blended; serve.