Drizzle 3 tbsp. olive oil in the bottom of a 9×13″ baking dish. I also recommend lining the bottom with parchment paper prior to adding the olive oil.
Rub your hands with a little olive oil, then gently remove the dough from the sides of the bowl to transfer to the prepared baking dish. Gently stretch the dough from underneath to fit the pan.
Let the focaccia sit in a warm environment (room temperature) for 4-6 hours, until nearly doubled in volume and super jiggly.
To Bake: Preheat oven to 425F for at least 30 minutes.
To Prep the Focaccia for Baking: Drizzle the focaccia with 3 tablespoons of olive oil, then evenly sprinkle all the finely chopped pistachios and walnuts. Dust with ground cinnamon all over.
Use your fingers to press into the dough to create dimples all over the surface. You'll want to also press in the nuts as much as possible to prevent them from burning on top. Add flaky salt.
Bake for 25-30 minutes, until golden on top. Let cool at least 20 minutes before slicing.
To Make the Simple Syrup: While the focaccia bakes, make the citrus simple syrup by combining the water, sugar, honey, and orange zest in a large saucepan over medium high heat. Stir until the sugar is dissolved.
Continue stirring until the sugar reaches about 200F, a deep caramel color, about 5-6 minutes. Stir in the vanilla.
Pour over the warm focaccia, then let sit for at least 20-30 minutes before slicing. If desired, drizzle with pistachio cream to enjoy!