In a large bowl, begin by adding the flour, salt, sugar and instant active dry yeast. While you could use a stand mixer to do this, we don't think it is worth the hassle of even getting it out for a dough that literally only needs to be stirred for one minute.
Slowly pour in water and stir with a dough whisk or wooden spoon until just combined. You can add less or more water depending on how wet the dough is. It should be a little sticky but not watery.
Cover the bowl with several clean tea towels or with plastic wrap and a tea towel over it and let the focaccia dough rise at room temperature or warmer, for at least 1 hour. Be sure not to put it in a drafty or cold place because it will not rise properly, instead look for a warm place like under a light, near a heater or in the oven with just the oven light on (no heat).
Drizzle olive oil on a 9 x 13 in dish (a large round baking pan will also work) and spread it with your hands until the entire dish is coated.
After the focaccia dough has risen, pour the dough gently into the greased baking dish. Put some olive oil on your hands so they won't stick to the dough (it will be a bit sticky) and push the dough gently until it fills the entire dish evenly.
When finished, cover with plastic wrap and let rise for another 15 minutes in a warm place.
Preheat oven to 400 ° F.
Rinse the rosemary sprigs and remove the leaves and let air dry on a paper towel.
After you have let the focaccia dough rise again in the baking pan, gently press your fingers over the dough to make indentations.
Drizzle the rosemary focaccia dough with a little olive oil and then sprinkle with rosemary leaves and salt flakes.
Once the oven temperature has been reached, bake in the lower part of the oven for about 20-25 minutes. Check to see if its ready by looking through the oven window, without opening it. It will have puffed up and have a golden brown crispy crust when done.
When baked, remove it from the oven and enjoy it with your favorite soup or salad with Italian seasoning or as sandwich bread. Out of all your recipes for focaccia, this is sure to become your favorite!