How to Make It
Preheat the oven to 400°. Lightly grease a 2-quart shallow baking dish. In a large saucepan, combine the potatoes with enough water to cover by 2 inches. Bring to a boil and cook until fork-tender, 15 to 20 minutes. Drain well and transfer to a large bowl. Mash the potatoes with 4 tablespoons of the butter, the crème fraîche, parsley and chopped sage. Fold in the Fontina and season with salt and pepper. Spread the potato mixture in the prepared dish in an even layer.
In a small bowl, using your fingers, blend the remaining 2 tablespoons of butter with the breadcrumbs and Parmesan until coarse crumbs form. Top the potatoes with the breadcrumb mixture. Bake for about 30 minutes, until golden and crisp on top.
Meanwhile, in a small skillet, heat 1/4 inch of olive oil over low heat. Add the sage leaves and cook, stirring occasionally, until crisp, about 3 minutes. Transfer the sage to paper towels to drain. Scatter the fried sage over the casserole and serve.