In a large bowl combine the flour, water, salt and yeast and mix until combined. 

Cover the bowl with plastic wrap and let it sit overnight at room temperature for 18 hours.

When the dough is ready it will be sticky and bubbly and have doubled in size. 

Evenly coat the bottom and sides of a 9 x 13 baking pan with the olive oil. 

Pour the dough out of the bowl and into the baking pan, turning it to coat with the olive oil making sure to bring the dough to the corners of the pan. 

Cover the baking pan with plastic wrap and let the dough rise for an additional 3 hours or until you see large bubbles forming in the dough. 

Preheat the oven to 425 F. 

Put a little olive oil on your fingertips and carefully poke the dough.

Slowly spoon the marinara sauce on to the dough {you do not need to cover the entire dough with sauce}. 

Sprinkle cheese over the sauce and then add the pepperoni slices. 

Bake for approximately 25 minutes or until the focaccia has a nice golden color and the edges are pulling away slightly from the pan. 

Cool 3-4 minutes before slicing.