Take out the ribs and thoroughly pat dry with a paper towel.
In a small bowl, mix all the spices—salt, ground mustard seed, ground white pepper, cayenne pepper, onion powder, and the sugar substitute.
Dip the ribs one by one into the mixed spice bowl until all sides of the ribs are generously covered with the spice mix (you should still have a bit of spice mix left).
Place ribs on a cutting board and rest.
Divide the olive oil evenly over the bottom of a minimum 6-quart Crock Pot-type-slow-cooker.
Divide the slices of one yellow onion on top of the olive oil.
Season with a few pinches of salt.
Place the seasoned short ribs in the slow cooker on top of the sliced onions. I like to arrange them on their side.
Tuck the remaining sliced onions in between the seasoned ribs. Fill up the spaces evenly in between the ribs.
Sprinkle any remaining spice mix evenly on top of the short ribs and the onions.
With the side of a knife flatten the peeled garlic.
Add on top of the meat in the slow cooker.
Top off with fresh Rosemary.
Set cooker on high, cover, and cook for 6 hours.
When the ribs are tender, serve with some cooked onions on top and spoon some of the liquid in the slow cooker over the ribs.