FOR THE FILLING:

1 1/2 lb. firm Golden Delicious or McIntosh apples (about 3 med.-lg. apples)
2 tbsp. butter
2 tbsp. plus 2/3 c. sugar
2 lg. eggs
2 tbsp. flour
8 oz. sour cream (scant cup, if measuring)
1/2 c. heavy cream
1 tsp. pure vanilla extract
Grated zest of 1/2 lemon
1/4 tsp. nutmeg (preferably freshly grated)
10" partially baked pie shell (preferably in glass pan)

FOR THE STREUSEL TOPPING:

1 c. walnuts (4 oz.)
1/2 c. all-purpose flour
1/3 c. packed dark brown sugar
1/2 tsp. ground cinnamon
1/8 tsp. salt
4 tbsp. unsalted butter, ice cold & cut into 1/4" bits

Peel and core the apples, the chop them into 1/2 inch pieces; you should end up with about 3 1/2 cups lightly pressed. Over a moderately high flame, heat the butter to sizzling in a skillet. Add the apples and cook for about 4 minutes, or until just tender, stirring frequently. Sprinkle on 2 tablespoon of sugar and toss for about 1/2 minute, or until the apples appear glazed, then remove the pan from heat.

In a mixing bowl, beat the eggs well, then add the 2/3 cup sugar and 2 tablespoons flour and continue to beat until blended. Whisk in the sour cream, heavy cream, vanilla, lemon zest and nutmeg. Turn the cooked apples into the partially baked pie shell and pour the egg/cream mixture over them. Bake the pie on a rack in the center of the oven at 350 degrees for 25 minutes, or until the center is thickened but not yet set.

Meanwhile, turn the walnuts into the jar of a food processor fitted with the metal chopping blade and pulse 2 or 3 times, or just until they are reduced to 1/4 inch pieces. Dump walnuts out into a bowl. Turn flour, brown sugar, cinnamon and salt into processor, blend for about 5 seconds, then add butter and pulse 3 or 4 times, or just until the mixture turns into a very coarse meal. Return the walnuts and pulse once, just enough to blend. Sprinkle streusel over pie, heaping it in the middle, and bake for about 20 minutes longer - that is, about 45 minutes total. Set the pie on a rack, serving it either slightly warm or at room temperature. (8 servings.)