Dissolve the yeast in the warm water (the water should not be too hot or it will deactivate the yeast).
In the bowl, mix the flour and semolina (you can use just flour, the semolina gives the dough a little more texture and crunch). Add the salt, oil and water/yeast mixture. Mix coarsely and transfer the dough to a floured surface.
Work the dough with the palm of your hand, kneading for about 10 minutes.
Cover it with a clean cloth and let it rest for 45 minutes in a warm place.
Oil the dish and flour it with semolina (or flour). Place the dough and gently stretch it to the edges
Make wells with your fingertips all over the surface of the dough. Brush with olive oil, and sprinkle with toppings if desired.
Cover again and let stand another 45 minutes.
Preheat oven to 200°C (400°F).
Bake the focaccia for 20 minutes.