Pat beef shanks dry and season with salt and pepper
Heat some oil in a pan and sear both sides thoroughly, about 4 minutes per side
Place onions in the bottom of the slow cooker. Add the root beer, paprika, apple cider vinegar and tomatoes. Stir to combine then add beef shanks
Cook on low for 4-5 hours
Remove shanks and set aside. Tent foil over plate to keep the shanks warm
Pour the gravy in a pan and boil until it's reduced by about a third - roughly ten minutes. Dissolve corn starch into water and add to gravy. Simmer for another 5 to 8 minutes. Strain into a bowl or gravy boat. Line platter with strained tomatoes and onions from the gravy and add the shanks