Start off by finly slicing all the onions.
Heat a braiser or dutch oven over medium-low heat.
Melt butter and add olive oil.
Add in onions and mix to coat.
Simmer for 10 minutes until they start to soften a bit and release some moisture.
Stir in salt and sugar.
Slowly cook onions for 30-40 minutes stiring often and scraping brown bits of the pan to incorporate back into the onions. We want the onions to cook very slowly so they dont burn.
Cook until the onions are carmalized and a deep brown color.
While onions are cooking bring beef stock to a gentle boil in a separate pan. Reduce to low.
Once onions are done add in the flour and mix well. Cook for 3 minutes or until all the flour is completely incorporated.
Slowly add in the beef stock, scraping the pan in the proess. Taste and add salt as needed.
Add in the white wine and bring to a gentle simmer.
Simmer partially covered for 45-50 minutes or until the liquid reduces and starts to thicken. Stir often.