In heavy-bottomed skillet or Dutch oven, melt butter over medium-high heat. Add olive oil and sliced onions. Season with salt. Cook, stirring often, for 10 minutes.
Reduce heat to medium; cook additional 20 minutes, until onions are caramelized. Remove from heat and cool. In bowl of electric mixer, combine cream cheese and sour cream until smooth. Add caramelized onions, cayenne pepper and fontina; mix well. Adjust seasonings to taste. Cover and chill 30 minutes before serving. Serve with crackers or raw veggies.
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