1. Heat broiler. Set top oven rack about 4 inches below broiler.
2. Cut the crusts off slices of bread. Cut each bread slice into 4 equal strips. Wrap 1 bread strip halfway around one grape and poke a long metal skewer through bread and grape so that skewer is pushed through both ends of bread strip with grape in the middle. Repeat with remaining bread strips and grapes.
3. Place all 16 skewers on a large baking sheet. Broil for 2 to 3 minutes, or until bread is browned and crisp, turning once halfway through broiling. Set aside.
4. In a small saucepan over medium-high heat, bring water and shallot to a simmer. Cook, uncovered, until shallots are tender, about 2 minutes.
5. Meanwhile, in small bowl, mix arrowroot with 1 ounce Champagne. Pour remaining Champagne into saucepan with the shallot. Whisk arrowroot mixture into the saucepan. Continue to cook and whisk until thickened, about 30 seconds.
6. Reduce heat to low and add Jarlsberg, whisking constantly until melted. Add pecorino and continue whisking until melted. Repeat with brie. Season to taste with salt and a pinch of cayenne. Pour mixture into a fondue pot and serve with bread and grape skewers for dipping.
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