While short ribs can be cooked in a crock pot, we suggest sous vide cooking as the long cooking time ensures complete conversion of the connective tissue and the consistenly low temperature ensures moist and tender meat protein.
Note that with the two to three day cooking time, you wouldn't normally prepare the sauce until the meat is ready.
Meat
Rub 750g flanken rib strips with salt and pepper, seal in bags, and submerse for 72 hours at 131°F (or 36 hours at 150°F for more fall-off-the-bone texture.
Remove meat from bags, conserving juice.
Optionally slice meat, separating between ribs.
Dry the meat, rub with olive oil, and briefly sear on a very hot cast-iron surface.
Onions
Fry onion until it is browned, add water and simmer until boiled away, then add pepper and remove from heat.