In a large mixing bowl, combine the warm water, sugar, and yeast. Let it sit for about 5-10 minutes until it becomes frothy.
Once the yeast is activated, add the flour, salt, and olive oil to the yeast mixture. Stir with a wooden spoon or spatula until a dough forms.
Knead the dough on a lightly floured surface for about 5 minutes until it’s smooth and elastic.
Place the kneaded dough back in the mixing bowl, cover with plastic wrap or a damp kitchen towel, and let it rise in a warm place for about 30 minutes or until it doubles in size.
While the dough is rising, preheat the oven to 400°F (200°C).
Once the dough has risen, punch it down and transfer it to a parchment-lined baking sheet. Gently stretch and flatten the dough to cover the sheet.
Arrange the thinly sliced apples evenly over the dough, pressing them lightly into the surface.
In a small bowl, mix the cinnamon and brown sugar together. Sprinkle this mixture evenly over the apple-topped focaccia. Drizzle with a little olive oil and add coarse sea salt if desired.
Bake in the preheated oven for about 25 minutes, or until the focaccia is golden brown and the apples are tender.
Remove from the oven and let it cool on a cooling rack for a few minutes before slicing. Serve warm or at room temperature.