Brown the beef bones in a medium sized skillet over medium heat in the vegetable oil. Season with salt and pepper to your taste (I used about 1 teaspoon of salt and 1 of pepper).
When the beef bones are browned (approximately 10-15 minutes), transfer them to the slow cooker.
Add in the vegetables, parsley and bay leaf (if you're using one).
Cover with water. At the this point, add a little more salt if needed (to your taste).
Cook on high for about 3 hours and skim the any foam that comes to the top.
Once it is simmering at a low boil (mine was at about 3 hours), change the temperature to low, and cook for about another 5-7 hours (I cooked mine total 8 hours).
Remove the parsley, bay leaf, vegetables and the bones; strain the broth.
Refrigerate overnight and the next day remove any fat that has solidified on the surface.