Begin by making the dough. In a large bowl, whisk together the flour and salt. Cut the butter into 1-inch pieces and toss in the flour. Make sure all the butter is coated in flour, and use your hands to press the butter into flat pieces, keeping some in large chunks, and others in smaller pieces. The dough should be rough, but not crumbly.
Add in the ice water and continue to press and mix the dough with your hands, it should start to come together. Move it to a work surface and press the dough down until is is about 2 inches high. Use a bench scraper to cut the dough into 3 pieces, pile the pieces on top of each other, and press them down. This is creating more lamination in the crust! Press the dough down into another 2-inch high mound, rotate 90 degrees, and repeat the same process.
After doing that a few times, press the dough down until it is about 1-2 inches thick, and wrap it in cling wrap. Use a rolling pin to roll out the dough into the edges of the cling wrap, forming a tight rectangle.
Store the dough in the freezer for at least 30 minutes while you prepare the rest of the ingredients.
In a small bowl, mix together the coconut oil, brown sugar, cinnamon and flour until it forms a smooth paste. Set aside.
Slice the peaches into uniform slices, add to a bowl and drizzle with the juice from 1/2 of the lemon. Toss and set aside.
Take the dough out of the freezer. On a floured surface, roll out the galette dough. Do this by gently hitting it with your rolling pin to soften it, and rolling in alternating directions until the dough is about 1/2-centimeter thick. Form it into a rough circle (feel free to trim the sides if need be) and transfer it to a piece of parchment paper. Feel free to add more flour as needed during this process.
Use a spatula or knife to evenly spread the cinnamon topping across the galette dough, stopping about 2 inches from the edges.
In a circular pattern, layer the peach slices on top of the cinnamon topping. Fold over the galette's edges to form a crust.
Preheat the oven to 375°F. While the oven is heating, put the galette back in the fridge to chill one last time.
When the oven is ready, whisk the egg in a small bowl, then use a brush to brush the galette crust with egg wash. Sprinkle the crust with flax meal.
Bake at 375°F for 35-40 minutes, or until the crust is a deep golden brown.
While the galette is baking, make the blueberry topping. In a small sauce pot or pan, add the blueberries, 2 tablespoons of water and a sprig of rosemary. Bring this to a slow simmer, mixing occasionally. Let the blueberries reduce until they are soft and jammy. Remove the rosemary sprig and transfer the blueberries to a bowl. Add the juice from the second half of the lemon and stir. Set aside.
When the galette is out of the oven, let it cool slightly before adding the blueberries on top. Feel free to get decorative! You'll have some blueberry juices left over, use that for serving.
After that, slice, serve and eat with ice cream or whipped cream.