Heat the butter and 1/4 cup of oil in a large saute pan over medium heat. Add the onions, cayenne pepper (optional), salt, and pepper and saute for 10 minutes.
Reduce the heat to medium-low and cook, stirring occasionally, for 20 more minutes until the onions are browned and caramelized. Allow the onions to cool.
Place the fromage blanc, sour cream and 1/2 cup of olive oil in the bowl of an electric mixer fitted with the paddle attachment and beat until smooth. (Can be done by hand if dairy is pre-softened.)
Add the onions and mix well. Taste for seasonings. Serve at room temperature.