I had some limes and pasta frolla to use up yesterday, and tried a little lime curd tart. I adapted the lemon curd recipe in Bernard Clayton's pastry book. The idea was to make the filling very intense (lots of grated peel), and spread just a half inch or so over a relatively thick, like 1/4 inch, crust, so that the sweeter, blander crust would cut the sourness of the curd. Worked pretty well, and I'm thinking about trying it with grapefruit. The thinness lets you cut tiny wedges and serve like a cookie.
I've had commercially-made lemon tarts that had a firmer filling. The standard recipes seem to be eggs-butter-sugar. Any innovations there?