Combine the strawberries, sugar and fennel seeds in a bowl. Stir to combine and allow to sit for 4 hours or, ideally, overnight.
Transfer the macerated strawberries and all of the liquid to a medium saucepan. Squeeze in the lemon juice and bring to a simmer over a high heat. Cook for 10-12 minutes or until thickened and the mixture reaches 100-101C on a thermometer. Allow to cool.
Meanwhile, make the pastry by mixing the flour, sugar and a good pinch of salt together in a large bowl. Use your fingertips to rub the butter into the flour. Don’t overwork the butter to small pieces; some larger flat pieces help to create the flakiness. Drizzle in the vinegar and enough of the iced water to just bring the dough together (you may not need all of it). It will still be shaggy, but should hold together when pressed and not be dry or floury. Flatten into a thick disc about 10cm-12cm in diameter, wrap and refrigerate for at least 1 hour.
Preheat the oven to 190C fan-forced (210C conventional). Line a round, 30cm-diameter baking tray with baking paper and set aside.
Roll out the dough on a lightly floured work surface to form a large circle about 3mm thick. Carefully lift the pastry and drape it over the prepared tray lined with baking paper.
Spread the cooled strawberries onto the pastry, leaving a 4-5cm border. Fold the overhanging pastry towards the centre, pinching the joins as you go to seal in the filling and form a galette. Whisk the egg with 1 teaspoon of water in a small bowl, then brush the egg wash over the pastry’s edge. Sprinkle the pastry with demerara sugar.
Bake in the preheated oven for 40-45 minutes or until the pastry is golden and the strawberries are bubbly. Cool.
For the rose cream, combine the cream, icing sugar and rosewater in the bowl of a stand mixer fitted with a whisk attachment and beat until soft and voluminous. Alternatively, use a normal whisk or handheld electric beaters. Serve with the tart.