For the dough: In the bowl of a stand mixer, whisk together the flour, yeast and salt. Make a small well in the center and then add 2 tablespoons oil and the water. Using the paddle attachment, mix on low until combined. Remove the dough to a flour-dusted work surface and knead for a few minutes until shiny and smooth.
Place the dough in a lightly oiled bowl, cover with plastic wrap and set aside in a warm, draft-free place until doubled in volume, about 1 hour. Remove the dough from the bowl and gently punch it down. Coat a 10-inch cake pan with 3 tablespoons oil. Place the dough in the pan and poke it all over with your fingers. Cover again with plastic wrap and set aside in warm place until puffy, about 45 minutes.
For the pizza: Preheat the oven to 425 degrees F. Spread the focaccia with the marinara sauce, then scatter with the meatballs. Tear the mozzarella slices and scatter them over the meatballs. Sprinkle with the Parmesan. Cover the pan with foil and bake for 15 minutes. Remove the foil and bake an additional 5 minutes, until the cheese is golden brown and bubbling and the crust is golden.