Preheat the oven to 325° Fahrenheit, no need to grease the pan, but use either a 9x9 square, 8x8 square, or pie pan. If using a food processor, pulse together the flour, sugar, and salt, then add in the butter and lemon zest and pulse until dough comes together. Dump mixture into a bowl and knead in the poppyseeds by hand until fully distributed. If mixing by hand, whisk together the flour, sugar, salt, and zest, then add in chunks of cold butter and knead them in until fully combined. Add in poppyseeds and knead until distributed.

Dump dough into your pan and press down until even (be sure to fill the entire pan, up to the corners). Bake 40 minutes for a 9x9 pan, or about 45-50 minutes for an 8x8 or pie pan. Let the shortbread cool a bit before cutting (in the pan) and carefully lifting out each shortbread slice!