How to Make It
Preheat oven to 400°F. Cook potato gnocchi according to package directions; drain and transfer to a large bowl.
Melt 2 Tbsp. unsalted butter in a large Dutch oven over medium-high. Add ham and cook, stirring often, until ham is lightly browned and any moisture has evaporated, 5 to 6 minutes. Using a slotted spoon, transfer ham to bowl with gnocchi. Wipe Dutch oven clean.
Melt 4 Tbsp. unsalted butter in Dutch oven over medium-high. Whisk in all-purpose flour. Cook, whisking constantly, until mixture is light golden brown, 1 to 2 minutes. Slowly whisk in whole milk, and bring to a boil; whisk vigorously to work out any lumps.
Whisk in kosher salt and black pepper. Remove from heat, and slowly whisk in shredded Gruyère cheese and 1/4 cup grated Parmesan cheese, until melted. Pour cheese sauce over ham and gnocchi; stir in thawed frozen green peas.
Transfer mixture to a lightly greased (with cooking spray) 11-x 7-inch baking dish. Sprinkle remaining 1⁄4 cup grated Parmesan cheese on top, and bake in preheated oven until bubbly and golden brown, 20 to 25 minutes. Remove from oven, and serve warm.