Mix all the ingredients the night before (you will prepare the dough in the morning).
Combine the water, psyllium husk flakes, and honey. Mix well and set aside for 5 minutes (it will have a jelly-like texture). Now add the preferment to the wet mix. Combine with a wooden spoon.
Mix all the dry ingredients in a glass bowl. Add the wet mix to the dry ingredients, incorporate as much as possible with a wooden spoon, then start mixing with your hand until the dough is homogeneous and has no lumps (soft and sticky).
Mix with a plastic dough scraper, scraping the dough from the sides of the bowl, and folding it into the center. Rotate the bowl and repeat with the same movement, for about 1 minute. Cover the bowl and let it rest for about 10 minutes.
Add the olive oil, mix with the hands, so the oil is well incorporated, then repeat the folding method using the plastic scraper. Let the dough rest for 30 minutes.
Transfer the dough to the pan, drizzle olive oil, and spread it out with your fingers. Cover the pan with a damp towel, put everything in a plastic bag, tighten it and let it proof/ferment for about 1h 30 minutes (at 72F).
Preheat the oven to 240C/465F for at least 40 minutes before baking. Garnish the dough with olives, rosemary, and sea salt flakes. Turn the heat down to 230C/450F and bake for 25 minutes.
Rest for 10 minutes, slice into squares, and serve. The focaccia can also be frozen.