Heat canola oil in a 12" cast iron skillet over medium-high heat until it just barely starts to smoke.
Cook the sausage in the hot skillet, browning on both sides.
Reduce the heat to medium and add the sliced onions. Stir to coat with oil, then cover the skillet with a lid.
Stir onions frequently and cook until the sausages are cooked through and the onions are soft.
Remove sausages from the skillet and keep warm.
Add the flour to skillet and stir with the onions to combine.
Add the white wine and reduce until it is almost completely evaporated.
Add the beef stock and reduce until the gravy starts to thicken.
Taste for seasoning and add salt & freshly ground pepper to taste. Pour and juices from the sausages back into the skillet.
Serve sausages, onions and onion gravy over the top of mashed potatoes.