Drain beets well; discard liquid or save for another use. Coarsely chop beets; reserve 1/2 cup for Pink Lemonade (see recipe to follow).
In large bowl, toss together slaw mix, mango and onion. Add vinaigrette; toss to coat well. Cut baguette lengthwise in half. Remove insides, leaving 1/2-inch shell on top and bottom. Spread bottom half with goat cheese. Toss beets with slaw mixture; spoon half over goat cheese. Arrange beef over slaw; spoon remaining slaw over beef. Close sandwich and press firmly.Wrap tightly with aluminum foil; refrigerate up to 4 hours before serving. To serve, cut baguette into 6 pieces.