French Onion Soup Potato Gratin

Name: French Onion Soup Potato Gratin
Ingredients: 8 tablespoons unsalted butter, wrapper reserved; 5 large Spanish onions, sliced about 1/4 inch thick; Kosher salt and freshly ground black pepper; About 3/4 cup beef stock; 2 tablespoons sherry; 2 teaspoons Dijon mustard; 3 pounds Yukon gold potatoes; 1 pound Gruyere, shredded; 8 ounces mozzarella, shredded; 1 1/4 cups heavy cream; 1 1/4 cups beef stock; One 4 1/2-ounce bag premade croutons, lightly crushed; One 3 1/2-ounce bag French fried onions