Preheat oven to 350 degrees F (175 C). Lightly grease a 9" x 13" baking sheet and line it lengthwise with parchment paper (this makes the bars super easy to lift out).

Blend the raspberries with a tablespoon of water and strain through a sieve to remove seeds. Set aside.

To make the crust, combine butter, flour, salt and sugar until smooth (I did this by hand, but you can also use a standing mixer). Press into the bottom of the prepared pan. Bake just until the crust begins to turn golden, 15 to 20 minutes.

While the crust is baking, make the filling by combining the sugar and flour. Whisk in the eggs and lemon juice. Add raspberry puree about a tablespoon at a time until the mixture is lightly pink. (Some raspberries are darker than others, so the amount you'll need will vary.) Pour filling over prepared crust.

Bake until outer edges begin to pull away from the pan and are lightly browned, 25 to 30 minutes. (The center of the bars may still be a bit wobbly but will firm as they cool.) Allow to cool completely before slicing into triangles for serving.