Melt the butter in a large skillet over medium heat. Add the liver and onions, and cook, stirring occasionally, until soft and browned on the edges, about 10 minutes. 

Add the wine, garlic, mustard, lemon juice and rosemary. Bring the mixture to a simmer and cook uncovered until the liquid has evaporated, about 10 minutes. Cool for 20 minutes. 

Place the warm liver mixture into a food processor and add the butter and creme fraiche. Process until smooth. 

Spoon the liver mixture into a crock or mold and chill well. At lest 4-5 hours. Serve with sourdough bread or crackers.