For the pastry:

3 packages (1 lb. each) frozen puff pastry (6 sheets total), such as Pepperidge Farm brand
Flour for rolling
For the mushrooms:

2 large portabella mushrooms, stems removed
6 Tbs. olive oil; more for cooking
2 cloves garlic, minced
1/2 tsp. fresh thyme leaves, lightly chopped
1/2 tsp. very roughly chopped fresh rosemary
Coarse salt and freshly ground black pepper
For the onions:

4 Tbs. unsalted butter
3 medium onions (about 6 oz. each), thinly sliced
1/2 cup medium-dry sherry (I prefer Amontillado)
Coarse salt and freshly ground black pepper
For the spinach and cheese:

12 cups tightly packed spinach (about 14 oz.)
Pinch ground white pepper (optional)
2 Tbs. unsalted butter
6 oz. Maytag blue cheese (or other good-quality blue, such as Roquefort)
For the filets:

6 portions beef tenderloin, 5 to 6 oz. each (choose equal-size center-cut portions)
Coarse salt and freshly ground black pepper
About 3/4 cup olive oil; more as needed
For assembly:

2 eggs, whisked together lightly with 1 tsp. water
Flour
To cook the Wellingtons:

2 eggs, whisked together lightly with 1 tsp. water