Instructions

MAKE THE FLAKY PASTRY (prepare at least 1 hour ahead):
1. In large bowl, combine flour and salt.
2. Add cold cubed butter and shortening, then use pastry cutter or two knives to cut fat into flour until mixture resembles coarse breadcrumbs with some pea-sized pieces remaining.
3. Sprinkle ice water over mixture 1 tablespoon at a time, tossing with fork after each addition, until dough just begins to come together (should still look shaggy).
4. Turn dough onto clean surface and gently press into cohesive disc without overworking.
5. Wrap tightly in plastic wrap and refrigerate at least 30 minutes or up to 2 days.

PREPARE THE CARAMELIZED ONIONS (can be made 1 day ahead):
6. Heat butter and olive oil in large skillet over medium-low heat.
7. Add thinly sliced onions and stir to coat in butter mixture.
8. Cook slowly, stirring occasionally, for 45-50 minutes until onions are deep golden brown and very soft (reduce heat if browning too quickly).
9. Add brown sugar, white wine (if using), white balsamic vinegar, salt, pepper, and fresh thyme during final 10 minutes of cooking.
10. Continue cooking until onions are caramelized, jammy, and liquid has evaporated.
11. Cool completely before assembling pie.

MAKE THE CHEESE-CREAM MIXTURE:
12. In medium bowl, whisk together heavy cream, eggs, minced garlic, chopped parsley, thyme leaves, salt, pepper, and nutmeg until well combined.
13. Divide each cheese variety in half (3 oz Stilton + 3 oz Stilton, 3 oz Gruyère + 3 oz Gruyère, 2 oz cheddar + 2 oz cheddar).
14. Combine half of each cheese variety (3 oz Stilton, 3 oz Gruyère, 2 oz cheddar) and fold into cream mixture. Reserve remaining half of cheeses for layering.

PARBOIL THE POTATOES:
15. Bring large pot of salted water to rolling boil.
16. Add potato slices and parboil 5 minutes until just tender when pierced with knife but still holding shape.
17. Drain carefully and spread on clean kitchen towel to cool completely, about 15 minutes.

ASSEMBLE THE LAYERED PIE:
18. Preheat oven to 375°F (190°C). Generously butter 9-inch deep-dish pie dish.
19. Create first layer: Arrange half of cooled potato slices in overlapping concentric circles, starting from outside edge and working toward center.
20. Create second layer: Spread half of caramelized onions evenly over potatoes.
21. Create third layer: Sprinkle half of reserved cheese mixture (1.5 oz Stilton, 1.5 oz Gruyère, 1 oz cheddar) evenly over onions.
22. Pour half of cream-cheese mixture over layers, distributing evenly.
23. Repeat layering sequence: Remaining potatoes arranged in overlapping pattern, remaining onions spread evenly, remaining reserved cheeses sprinkled on top, remaining cream mixture poured over all.
24. Dot top with small pieces of butter distributed evenly across surface.

ADD THE FLAKY PASTRY LID:
25. On lightly floured surface, roll chilled pastry into circle approximately 11 inches in diameter and 1/8-inch thick.
26. Carefully drape pastry over rolling pin and transfer to cover pie filling.
27. Trim pastry to 1-inch overhang beyond edge of pie dish.
28. Fold overhanging pastry under itself and crimp decoratively around rim using fork or fingers, ensuring good seal all around.
29. Cut 4-5 small steam vents in center of pastry using sharp knife.
30. Brush entire top surface generously with beaten egg for glossy finish, being careful not to let egg drip over crimped edges (which can prevent proper rising).

BAKE TO PERFECTION:
31. Place pie dish on rimmed baking sheet to catch any drips.
32. Bake 50-60 minutes until pastry is deep golden brown, well-risen, and crisp.
33. Check internal temperature with instant-read thermometer - filling should reach 165°F (74°C) in center.
34. If pastry browns too quickly during baking, loosely tent with aluminum foil.
35. Remove from oven and let rest 15 minutes before cutting to allow layers to set and cream to thicken.
36. Garnish with fresh thyme sprigs before serving.

STORAGE AND SERVING:
Serve warm or at room temperature
Store covered in refrigerator up to 3 days
Reheat individual portions in 350°F oven for 15-20 minutes
Best consumed within 24 hours for optimal pastry texture