Preheat oven to 375 degrees. Set a mini-muffin tin out for shaping the shells.
Sift sugar and flour together into a small bowl. Add butter and juice (or water) and whisk until even and smooth.
Flip a large backing sheet over and place a piece of parchment paper across it. Spoon a heaping teaspoon of batter onto the parchment paper and, using the back of the spoon in a circular motion, spread the batter into a circle, 2 inches in diameter. Make 6 circles on the baking sheet, leaving space between them. (You will need to do several batches of the shells, baking them 6 at a time.) Bake until the edges begin to turn brown, 7-9 minutes.
Remove the baking sheet from the oven. Working quickly, use a small spatula or butter knife to lift one of the shells off the baking sheet and directly into a hole in the muffin tin. Gently press the center of the shell to form an indentation for filling. Continue with the remaining shells. Note: The shells will harden almost immediately after you lift them off the baking sheet. Work quickly, and don’t worry too much about making uniform shapes – part of the fun is that the shells come out in a variety of different shapes.
Continue baking and forming shells until all of the batter is gone. The shells can be removed after a minute in the muffin tin and placed on plate to cool while preparing the filling.
Fill tartlets with the following combinations and serve immediately (I found that they lose some of their crunch if not served immediately after being filled).