First, bring the butter to room temperature. You will want to be able to mix it with a spoon.
While the butter is softening, cook the garlic and the onion so that it isn't raw. You will want these to be cool before starting to mix the butter.
Now, in a bowl combine all of the ingredients together and mix them well.
Then, take some plastic wrap and roll the butter into a log shape. Another trick is to use a silicone cube tray as well but I personally like the log. Roll and shape the butter so that you can easily cut slices from it.
Place the butter back into the refrigerator for a few hours to give the butter a chance to harden again.
When ready to serve cut slices to cook with your next turkey.