It’s mid-afternoon. Six hours from now your dining room table will be bustling with friends waiting on your French onion soup for a dinner party. You’ve got 10 pounds of onions to slice, and your knife just glanced off the first onion as you tried to chop through the stem. Don’t panic. In less than 15 minutes, you can get your blade back to a razor fine edge. That’s true whether you want to take t...