To make the crust: Mix together the cream cheese, butter, sugar, and salt until smooth., Add the flour and mix gently until the dough is cohesive., Shape the dough into a 1"-thick disk, wrap it in plastic, and refrigerate for at least 1 hour., To make the filling: Whisk together the lemon juice, zest or oil, and the sugar., Whisk in the eggs, then the sour cream., To assemble: Roll the dough on a floured surface into a circle 1/8" thick., Using a 2 3/4" biscuit cutter, cut out 30 circles, gathering the scraps and re-rolling the dough as necessary., Preheat the oven to 375°F. Grease a 24-cup mini-muffin pan., Transfer dough circles to the cups of the pan, pressing into the corners and up the sides of each cup. You'll have six circles left over; refrigerate them and bake them as a second batch, once you've finished this first batch and allowed the pan to cool to just barely warm., Chill the tart shells in the refrigerator until firm, about 20 minutes., Remove the tart shells from the refrigerator then pour the filling into each shell until it nearly reaches the rim of the cup., Bake the tarts for 20 to 24 minutes, until the crust turns golden and the filling has set., Remove the pan from the oven and let the tarts cool for 10 minutes in the pan., Transfer the tarts to a rack to cool completely before serving. Let the pan cool, then bake the remaining six tarts.