1) For the crust: Mix together the cream cheese, butter, sugar, and salt until smooth.
2) Add the flour and mix gently until the dough is cohesive.
3) Shape the dough into a 1"-thick disk, wrap it in plastic, and refrigerate for at least 1 hour.
4) For the filling: Whisk together the lemon juice, zest or oil, and the sugar.
5) Whisk in the eggs, then the sour cream.
6) To assemble: Roll the dough on a floured surface into a circle 1/8" thick.
7) Using a 2 ¾" biscuit cutter, cut out 24 circles, gathering the scraps and re-rolling the dough as necessary.
8) Preheat the oven to 375°F. Grease a 24-cup mini-tart pan or mini-muffin pan.
9) Transfer the dough circles to the cups of the pan, pressing into the corners and up the sides of each cup.
10) Pour the filling into each of the tart shells until it nearly reaches the rim of the cup.
11) Bake the tarts for 28 to 30 minutes in the mini-tart pan, or 18 to 20 minutes in the mini muffin pan, until the crust turns golden and the filling has set.
12) Remove the pan from the oven and let the tarts cool for 10 minutes in the pan.
13) Transfer the tarts to a rack to cool completely before serving.
Yield: 24 mini tarts.