Preheat oven to 400 degrees F. Place potatoes in a large pot or Dutch oven, cover with cold water by 2-inches, add 1 tablespoon of salt and bring to a boil. Reduce heat, simmer until just tender, about 15 minutes.

Place half-and-half in a small saucepan and bring to a simmer over low heat. Drain potatoes well, return them to pot and place back on burner over low heat to dry out potatoes and get out any additional water. Process potatoes through a ricer or food mill set over a large bowl (or use traditional potato masher). Stir in warmed half-and-half and butter until combined, fold in 3/4 cup of the Fontina cheese, 3/4 cup of the Gruygre cheese, and season generously with salt and white pepper, to taste. Scrape mixture evenly into a buttered 9- by 13-inch baking dish and sprinkle the remaining 1/2 cups of Fontina and Gruyere cheeses evenly over the top. (This recipe can be done ahead to this point; let casserole cool for 30 minutes, then refrigerate until cool. Cover with plastic wrap, and refrigerate for up to 24 hours, or freeze for up to 1 month. Remove from refrigerator 30 minutes prior to cooking. Increase baking time to 30-35 minutes.)

Cover with foil, lightly sprayed with cooking spray, bake for 20 to 25 minutes. Remove from oven and preheat broiler. Remove foil and broil until cheese is lightly browned and bubbly, about 5 minutes.

Makes 10 to 12 servings.