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
Preheat oven to 350°F. If using 14-oz. package of puff with 1 sheet of pastry, roll pastry to a 18x13" rectangle on a lightly floured surface. Cut in half crosswise; reserve remaining piece for another use. If using 17-oz. package with 2 sheets of pastry, roll 1 sheet to a 13x9" rectangle on a lightly floured surface; reserve remaining sheet for another use. Cut pastry lengthwise parallel to 13" side into 14 (1/2"-wide) strips. Transfer strips to a parchment-lined rimmed baking sheet and chill while assembling casserole. Bring a medium pot of water to a boil. Add 6 eggs, cover, and cook 7 minutes. Transfer eggs to a bowl of ice water and let cool completely. Peel eggs. Meanwhile, working in batches, cook bacon in a large skillet over medium heat, turning occasionally, until crisp, 10–12 minutes. Transfer bacon to paper towels, then transfer bacon fat to a small bowl. Let bacon cool, then coarsely chop. Heat 2 tsp. bacon fat in same skillet over medium. Add onion and cook, stirring occasionally, until onion softens and turns translucent, 5–7 minutes. Grease a 13x9" pan with 1 Tbsp. bacon fat; discard remaining fat or reserve for another use. Whisk 8 eggs in a large bowl. Add cheese, salt, pepper, nutmeg, and 2 cups cream and whisk to combine. Add potato, spinach, onion, and chopped bacon and stir to combine. Pour into prepared pan. Cut cooked eggs in half lengthwise and arrange over filling, then press down so top edge of egg is even with top of filling. Arrange chilled dough strips lengthwise 1" apart on the diagonal, trimming to fit flush against edge of pan; use trimmed dough on shorter sides. Lay more strips over arranged strips going the other direction, creating a lattice-like (but not woven) design. Brush dough with remaining 1 Tbsp. cream with a small pastry brush. Bake casserole, rotating pan halfway through, until pastry is deep golden brown and cooked through, 50–55 minutes. Let cool 15 minutes before serving. Do Ahead. Pastry strips can be cut 1 day ahead; arrange on rimmed baking sheet, cover with plastic wrap, and chill. Filling can be made 1 day ahead; cover and chill. Baked casserole can be made 1 day ahead; wrap tightly in plastic and chill. Reheat in a 350°F oven until warmed through, about 20 minutes.