Croque Madame and Tot Casserole

Name: Croque Madame and Tot Casserole
Ingredients: Cooking spray; 3 tablespoons unsalted butter; 3 tablespoons all-purpose flour; 3 cups half-and-half; 8 ounces fontina cheese, shredded (about 2 cups), divided; 2 tablespoons Dijon mustard; ½ teaspoon kosher salt; ½ teaspoon freshly ground black pepper; 5 ½ cups frozen potato tots (from a 32-oz. pkg.), divided; 6 ounces thick-cut sliced ham, torn or cut into 1 1/2-in. pieces (about 3/4 cup); 8 large eggs; 1 tablespoon chopped fresh chives