Mrs. Cratchit said she would give Scrooge, " A piece of her mind to feast upon, and I hope he'd have a good appetite for it." What does this mean? Why does Mrs. Cratchit say this?

Mr. Cratchit has just made a toast to his employer. Regarless of Scrooge's treatment of him, Bob Cratchit holds an optimistic viewpoint of humanity that most cannot understand. Especially, his wife. Notably, she like to tell Scrooge what she thinks of him......