Heat oven to 375°F.
Rinse potatoes.
Transfer potatoes to a large saucepan; cover with water.
Bring to a boil.
Cook, uncovered, for 2 minutes.
Drain and rinse potatoes with cold water; set aside.
Melt butter in a large skillet over medium-low heat.
Whisk in flour until light brown.
Gradually whisk in milk.
Bring to a boil; cook and whisk for 2 minutes or until thickened.
Reduce heat to low.
Gradually whisk in 1/2 cup each cheddar and fontina until smooth.
Remove from the heat.
Whisk in chives and Dijon mustard.
Layer potatoes in a greased 13 x 9-inch baking dish.
Season with salt and pepper.
Sprinkle with remaining cheddar and fontina.
Pour cheese sauce over potatoes.
Bake, covered, for 45-50 minutes or until potatoes are tender.
Uncover; broil 4 inches from the heat for 1-2 minutes or until top begins to brown.
