Set your slow cooker to high temperature. Add the butter and oil.
When the butter is melted, add the sliced onions, salt and pepper.
Cook the onions on high for about 6 hours, stirring a few times, until they have browned (caramelized) to the color you like.
Toward the end (maybe 5 hours into the cooking) because they are wet, vent the pot lid by putting chopsticks across either end to allow steam to escape (if you just prop one side, the condensation will just drip back into the pot).
When the onions are almost done, add the sherry, and continue to cook on high for another half an hour or hour.