Cut florets and peeled stems vertically into thin slices, slicing from stem to the floret top.
Toss sliced broccoli with the pecans and cranberries in a large bowl.
Whisk together buttermilk, mayo, vinegar, sugar and salt until smooth in a separate bowl.
Stir in onion and let mellow in the dressing for 10 minutes.
Pour dressing over broccoli mixture and add a generous amount of black pepper.
Stir the salad until the broccoli is evenly coated with the dressing.
Serve immediately, or keep covered in the refrigerator for up to 3 days.
Yields 8 cups