In a large bowl combine flour and salt, add the butter using a pastry blender, cut it into the flour until it resembles coarse crumbs. Add sour cream and work it into the mixture with your hands until it comes together. Knead briefly, wrap in plastic wrap and chill in the fridge for 30 minutes.
Preheat the oven to 400F/200C
In a large bowl combine fresh blackberries with sugar, flour and lemon zest by lightly tossing them taking care not to mash blackberries.
On a lightly floured surface roll out ⅔ of the pie crust to fit a 9" deep dish pie plate. It should be large enough so the sides hang over the edges of the plate. Drape the rolled out pastry on the rolling pin and carefully transfer it into the pie plate. Fill it with blackberry filling.
Roll out the remaining pastry and cover the pie with it. Pinch the sides of the pie really well and brush the top with lightly beaten egg. Cut slits in the pie and sprinkle with additional sugar if desired. Bake in the preheated oven for 45 minutes until golden.
Before serving swirl honey into creme fraiche and sprinkle with some lavender. (optional) Cool and serve with honeyed creme fraiche.