Place onion and butter in a Crock-Pot on LOW. Cook, stirring occasionally, until slightly caramelized.
Season beef shank with salt and pepper. Dredge in flour.
Heat oil to nearly smoking in a heavy skillet.
Brown beef shank on both sides, turning often.
Transfer to Crock-Pot.
Deglaze skillet with sherry. Pour over beef shank.
Add beef stock.
Go through your stores and see if you can't find anything else that belongs in the pot. I added a little miso and a rind of Grana Padano.
Cover and cook overnight on LOW.
Skim fat (save it though, because it tastes great).
Add mushrooms and potatoes. Cover and cook until tender.
Taste and adjust.
