Bacon and Egg Casserole

Name: Bacon and Egg Casserole
Ingredients: 1 (14- or 17-ounce) box of puff pastry (preferably all-butter puff), thawed according to package directions; All-purpose flour (for surface); 14 large eggs, divided; 9 strips bacon (about 8 ounces); 1 large onion, thinly sliced; 8 ounces Gruyère, coarsely grated (about 2 1/2 cups); 1/2 teaspoon kosher salt; 1/2 teaspoon freshly ground black pepper; 1/4 teaspoon freshly grated or ground nutmeg; 2 cups plus 1 tablespoon heavy cream, divided; 1 medium Yukon Gold potato (about 5 ounces), cut into 1/2 cubes; 5 ounces baby spinach