Gnocchi Gratin with Gorgonzola Dolce

INGREDIENTS

• 6 ounces Parmesan, finely grated, divided
• 1 cup all-purpose flour plus more
• 1 1/4"-thick slice fresh ginger
• 2 tablespoons (1/4 stick) unsalted butter, room temperature, divided
• 1 sprig rosemary
• 4 ounces Gorgonzola dolce or other mild blue cheese
• 1 pound russet potatoes
• 3 tablespoons fine dried breadcrumbs
• 1/2 teaspoon freshly grated nutmeg
• 1 large egg yolk
• 1 cup heavy cream
• 1 large egg
• 1 teaspoon kosher salt plus more