Trim broccoli and chop it by hand into smaller pieces, you can use the stem (peelin it first), or not, to your preference. Toss the broccoli with the almonds, cranberries and most of the red onion in a large bowl. Meanwhile, whisk the buttermilk, mayo, vinegar, sugar and a pinch each of salt and pepper in a smaller bowl. Pour dressing over broccoli (if you've skipped the stems, you might not want it use it all) and toss it well. Season well with salt and pepper to taste. This slaw should keep up to a week in the fridge.