In a large bowl whisk together the water and yeast. If using active yeast, let this sit for 5-10 minutes until foamy. If using instant yeast the waiting part can be skipped. Add the flour and salt and mix until no dry flour remains. The dough will look very sticky and shaggy at this stage. 500 g all-purpose flour, 420 g water, 2 tsp instant yeast, 1.75 tsp salt
Cover and let it rest for 10 minutes, then wet your hands and perform a set of stretch-and-folds: gently lift one side of the dough, stretch it upward and fold it over to the opposite side. Rotate the bowl 90 degrees and repeat on the next side until all four sides are folded. Let the dough rest 15 minutes and stretch and fold. Repeat 2–3 more times. Each set will make the dough smoother and stronger.
While the dough rises, peel and dice the apples into small cubes. Melt butter in a pan. Add the apples, brown sugar and vanilla. Cook for just 3 minutes until they begin to soften and release some moisture. Transfer to a bowl and let cool completely. 3 medium apples, 1 Tbsp butter, 1 Tbsp brown sugar, 1/2 tsp vanilla paste
In a small bowl mix the sugar and cinnamon together. 70 g granulated sugar, 2 teaspoons cinnamon
Generously coat a rectangular pan with olive oil. Turn the dough out into the pan and push into a rectangle. 3-4 tbsp olive oil
Sprinkle a 1/3 of the cinnamon sugar over the dough and scatter 1/3 of the cooled apple cubes on top, holding back any liquid from the apples. Fold the top third of the dough down, then the bottom third up, like folding a letter, to enclose the filling.
Turn the dough 90 degrees, sprinkle with another 1/3 cinnamon sugar and 1/3 of the apples, and roll it up into a thick spiral log. Place it lengthwise in the pan, then gently press and stretch it out to flatten it.
Cover loosely and let the dough rise at room temperature until puffy and jiggly, around an hour depending on room temperature.
Preheat the oven to 220°C / 425° F.
Once risen, scatter the remaining apples and cinnamon sugar evenly over the surface. With oiled fingers, press deep dimples all over the dough, pushing the toppings gently into place.
Bake for 25–30 minutes until the focaccia is deeply golden and the edges pull slightly from the pan. Tent with foil if the top browns too quickly.
Cool in the pan for 5 minutes then turn it out onto a cooling rack. Whisk together powdered sugar, milk and vanilla for a glaze, drizzle over the warm focaccia or brush it on with a pastry brush, then slice and serve. 90 g powdered sugar,1-2 Tbsp milk,1/2 tsp vanilla paste