Sift the dry ingredients into a large bowl, adding back any bits of grain or other ingredients that may remain in the sifter. Add the 1/2-inch pieces of cold butter.
Rub the butter between your fingers, breaking it into smaller bits. Continue rubbing until the butter is in sizes ranging from peas to hazelnuts. The more quickly you do this, the more the butter will stay solid, which is important for the success of the recipe.
Add the vinegar and 8 tablespoons of ice water to the flour mixture. Working from the outer edge of the flour, mix the ingredients with your hands just to moisten the flour. The dough needs to come together as mostly one lump, with a few shaggy pieces. Squeeze the dough together to see if a ball forms. If it is too dry to come together, add additional ice water 1 tablespoon at a time.
Pile the dough onto a sheet of plastic wrap, sprinkle a few drops of water over the top, wrap tightly, and chill for a minimum of 1 hour or overnight. Unwrap the dough onto a floured surface. Pat the dough into a square, then use a rolling pin to roll it into a rectangle about 81/2 x 11 inches. The dough will be crumbly and rough around the edges, but don't add more flour or water, as it will come together during the rolling.
For the first turn, fold the dough into thirds like a letter. The seam should be on the left side. Turn the dough so that the seam is at the top and parallel to your body.
For the second turn, again roll the dough into an 81/2 x 11-inch rectangle and repeat the previous step. For the third turn, repeat the previous step, and then wrap the dough in plastic and chill in fridge for 1 hour or up to 3 days before using, or freezer for up to a month. Thaw dough in fridge if frozen
In a large bowl, stir the strawberries and rhubarb with the granulated sugar, gently mixing until the sugar evenly coats the fruit. Set aside.
Place the chilled pastry on a lightly floured work surface. Roll it into a 13”-14" inch circle, about 1/4" thick. Lay the circle of dough onto a sheet of parchment paper. It will overlap the edges of the paper, but you will be folding it over later when you form the galette. Let the dough rest at room temperature, lightly covered, while you finish the filling.
Drain the strawberry-rhubarb mixture, pressing gently to remove any excess liquid; reserve the liquid for the glaze.
In a small bowl, combine the flour and brown sugar, breaking up any large clumps with your fingers. Add the flour/sugar mixture and lemon zest to the drained fruit. Toss gently until the fruit is evenly coated.
Pour filling into the center of the galette dough, leaving a 2 inch bare pastry border. Brush pastry border with egg wash.
Fold the uncovered border of dough up over the filling, allowing the dough to pleat as you lift it up and work your way around the galette. This sounds harder than it actually is. It will pleat naturally. Place galette onto a rimmed baking sheet and place into fridge for about 30 minutes.
Preheat the oven to 375°F. After galette has chilled, remove from fridge, brush pastry with egg wash and sprinkle with coarse sanding sugar. Bake for 20 minutes. Remove from oven and sprinkle crust with sliced almonds. Return galette to oven and continue baking for a further 10-15 minutes until galette has a firm, golden bottom crust, a bubbling filling, and a deeply coloured, crispy top crust. If in doubt, err on the side of baking the galettes a bit longer. They won’t dry out, and extra time in the oven will ensure a crispy crust.
While the galette is baking, prepare the glaze. Pour the reserved strawberry-rhubarb juice (about 90g, 1/3 cup plus 1 tablespoon) into a small saucepan. Add dried hibiscus flowers if using, Stir in the lemon juice and bring to a boil. Simmer the mixture until thickened, about 5 to 7 minutes; it should be the consistency of heavy cream. Strain glaze to remove hibiscus flowers.
Immediately after baking, brush the glaze onto the fruit-filled center of the galette. Let cool for at least 30 minutes before serving. Garnish with freshly whipped cream or ice cream, or simply enjoy plain.