Bloom the yeast in tepid water with 2 tablespoons of flour (subtracted from the total amount of flour predicted for this recipe). Whisk well and let it stand covered for 10 min.
In a large bowl mix all-purpose and whole wheat flour with salt and make a well in the middle. When the yeast has activated, start adding the wet ingredient to the center of the bowl, stirring with a wooden spoon. Add olive oil and stir well until a wet dough is formed. Make sure there aren't any pockets of flour in the dough.
Cover with a plate and let the dough rest for the first 30 minutes.
After the first 30 minutes, prepare a small bowl with water on the side, and to the first stretch & fold 4-6 times. Repeat this step altogether 3-4 times at intervals of 20-30 minutes.
Line a baking tray with parchment paper, pour a bit of olive oil, and coat the paper with it. Then, after the last stretch & fold (after the dough has rested), gently transfer the dough to the tray and stretch it to the edges. Cover with a lid or protect with plastic foil and transfer the dough to the fridge to rest for 8 to 14 hours.
The next morning, take the focaccia out of the fridge and let it rest at room temperature for 30-45 minutes. Half an hour before you put the focaccia in the oven, preheat the oven to 230°C (without the fan on). Pour the rest of the olive oil evenly over the dough and make dimples with your fingers.
Add the olives, sprinkle with seeds, and Fleur de sel, and put it in the oven until golden brown on top, about 25-30 min.
After the focaccia has been baked, take it out of the oven and immediately carefully remove it from the tray onto a cooling rack (without parchment paper so it doesn't steam from below). Let it cool all the way through (I know, this is the hardest part), and then enjoy!