Whisk eggs, yolk, sugar and lemon juice until well combined; whisk in double cream. Stir through rosemary and zest. Cover and refrigerate overnight for flavours to infuse.
Meanwhile, for rosemary pastry, process flour, icing sugar and a pinch of salt in a food processor; add butter and process until coarse crumbs form. Add egg, rosemary, zest and 2-4 tsp iced water and pulse until just combined. Lightly knead, then form into a disc, enclose in plastic wrap, and refrigerate for 1 hour (see note).
Remove pastry from fridge and set aside for 20 minutes. Roll out pastry on a lightly floured piece of baking paper to a 3mm-thick 6cm x 40cm rectangle, then line a greased 10.5cm x 34.5cm rectangular loose-based tart tin, trimming excess pastry. Reserve trimmed pastry, enclose in plastic wrap and refrigerate. Lightly prick base of tart shell with a fork; refrigerate for 1 hour.
Preheat oven to 180°C. Place tart shell on an oven tray. Line pastry with a large piece of foil and fill with pastry weights or rice. Blind-bake tart shell until edges are pale golden and side is set (30 minutes). Reduce oven to 170°C. Remove foil and weights, then bake until light golden and crisp (8-10 minutes). Repair any cracks or holes with reserved pastry. Brush with egg-yolk mixture and cook until crisp (5 minutes). Remove from oven.
Reduce oven to 150˚C. Remove lemon filling from fridge. Skim any foam that has formed on top, then strain filling through a fine sieve. Carefully pour half the filling into tart shell. Place in oven, then fill to the rim. Bake until set around edges but centre still has a slight wobble (20-25 minutes). Cool tart in tin for 2 hours, then remove from tin and cool completely (if you leave tart in tin for too long the pastry will sweat).
Whisk crème fraîche and icing sugar to soft peaks. Carefully slice tart and serve with crème fraîche and rosemary flowers. Tart is best eaten on day it’s made.