Hydrate the yeast. Mix yeast packet with the warm water and sugar, and let the foam double in size.
Make the brioche dough. Mix the flour, sugar, water, activated yeast, lemon zest, salt and eggs into the bowl of an electric mixer with a dough hook attachment and mix on medium speed for 1-2 minutes, until it's all combined. Then turn up the speed to medium high and let it run for about 5 minutes, or until the dough starts coming away from the sides and forms a ball. Turn off the mixer and let the dough rest for a minute. Slowly add the butter to the dough, about 25g at a time. Once it is all incorporated, mix on high speed for 5min until the dough is glossy, smooth and very elastic when pulled.
Proof the dough. Cover the bowl with saran wrap and leave to proof in the bowl until it has doubled in size - about 1-2 hours.
Shape the loaf and proof twice more. Pour the dough out onto a floured working surface, and sprinkle the blueberries + 1 tablespoon of sugar on top. Roll it up, and place it into a loaf pan, and place covered in the fridge overnight (8 hours if you're working during the daytime). For the final proof, pull the loaf out of the fridge and let it rest in a warm room (75F) for 1-2 hours until it's about doubled in size.
Bake. Preheat your oven to 350F and brush egg wash on top of the loaf, and sprinkle demerara sugar on top. Bake for about 45 minutes, until browned and pulling away from the sides of the pan a little bit.
The cook time doesn't include the resting/proofing time