Prepare the crust: Blend flour, powdered sugar, and salt in a food processor for 5 seconds.

Add chilled butter cubes to the flour mixture.

Process using on/off turns until the mixture resembles coarse meal.

Add chilled whipping cream.

Process using on/off turns until the dough comes together in moist clumps.

Gather the dough into a ball and flatten into a disk.

Wrap the dough disk and chill in the refrigerator for at least 1 hour, or up to 1 day.

Roll out the chilled dough on a lightly floured surface to a 12-inch round.

Transfer the dough to a 9-inch diameter tart pan with a removable bottom.

Trim any overhang to about 1/2 inch and fold it inward, forming double-thick sides.

Refrigerate the crust for 30 minutes.

Preheat oven to 375°F (190°C).

Line the chilled crust with foil and fill with dried beans or pie weights.

Bake for about 30 minutes, or until the sides are set and slightly golden brown.

Remove the foil and beans and continue baking for about 10 minutes, or until pale golden brown, pressing with the back of a fork and piercing if the crust bubbles.

Cool the crust in the pan on a wire rack for 30 minutes.

Prepare the filling: In a bowl, blend melted butter and dark brown sugar until smooth.

Whisk in the eggs one at a time, then add lemon juice, lemon zest, and cinnamon.

Stir in the dark raisins and chopped walnuts.

Pour the filling into the cooled tart crust.

Bake for about 30 minutes, or until the filling is deep brown and set in the center, covering the crust edges with foil if browning too quickly.

Let the tart cool completely at room temperature. (Can be made 1 day ahead; cover and let stand at room temperature).

Beat chilled whipping cream in a medium bowl until stiff peaks form.

Push up the pan bottom to release the tart.

Spoon whipped cream alongside the tart and garnish with toasted walnuts to serve.