Kale and Bacon Hash Brown Casserole

Name: Kale and Bacon Hash Brown Casserole
Ingredients: 1 tablespoon unsalted butter, at room temperature; 6 slices bacon; 1/2 medium yellow onion, finely diced; 4 cups lightly packed chopped kale (about 3 ounces); 1 garlic clove, minced; 12 large eggs; 1 cup whole milk; 1 tablespoon Dijon mustard; 6 ounces shredded mozzarella (about 1 1/2 cups); 6 ounces shredded Gruyère (about 1 1/2 cups); 1 (20-ounce) bag frozen hash browns, thawed and patted dry; Kosher salt and freshly ground black pepper