1. Make the filling. In the bowl of a food processor fitted with the metal blade, purée the lemon slices. With the motor running, add the eggs, one at a time. Add the sugar and process until well mixed. Transfer to a bowl, cover, and refrigerate for at least 8 hours or overnight.

2. Make the crust. In the bowl of a food processor fitted with the metal blade, pulse the walnuts, flour, and sugar until well mixed and ground. Add the butter and egg and mix until the dough comes together.

3. Press the dough in the bottom and up the sides of a 9-inch tart pan with a removable bottom. Refrigerate for at least 1 hour or overnight.

4. Preheat the oven to 350°F.

5. In a small bowl, stir together the sugar and flour. Add the butter and using a fork, pastry blender, or your fingers, work the mixture until it resembles coarse crumbs.

6. Pour the filling into the tart pan, sprinkle evenly with the topping, and bake for 45 to 50 minutes, or until the topping is golden brown and the filling looks set (it will be wobbly in the center).

7. Carefully remove the tart from the pan by removing the bottom of the tart pan. Serve warm, topped with sweetened whipped cream, if desire.