You can make them in the crock pot and freeze portions.
Slice 3 pounds of yellow onions.
Put a cup of butter in the bottom of your slow cooker.
Toss all the onions on top. Put the lid on. Turn your slow cooker on low and leave it to cook for 8 to 12 hours. 8 hours is the minimum but the longer they sit, the more they will caramelize. You can stir them once or twice if you would like to prevent any sticking but it is not necessary.
Freeze in a muffin tin, ice cube tray, or flat in a ziplock. You'll be able to break off a chuck.
Awesome on burgers, baguettes and sandwiches, french onion soup of course, pasta, and pizza.