For the dressing: In a bowl, mix the mayonnaise, vinegar, mustard, salt and pepper, and reserve.
For the coleslaw: In a large bowl, mix the cabbage, collard greens, carrots, onions and green onions.
Pour half of the dressing over the slaw and mix by hand. Add more dressing to taste.
Mix in the lemon juice and refrigerate for 2 hours before serving.
Season with salt and pepper before serving.