Step 1Heat the oven to 450 degrees. Remove the dishcloth from the top of one of the focaccias and, using your fingertips, gently tap down on the focaccia with about 5 light strokes to nudge it toward the edge of the pan; it still might not reach the edges, but don't worry.
Step 2One at a time, push the mozzarella cubes into the focaccia dough while simultaneously pushing outward to encourage the dough toward the edge of the pan, arranging the cubes evenly over the surface of the dough and pressing them so deep they are almost flush with its surface.
Step 3Keeping the onions in a chunk of layers, arrange the cubes in the dough with the cheese, pressing them down into the dough and slightly outward. At this point, the dough should be touching the edge of the pan.
Step 4Brush the surface generously with olive oil, then sprinkle over the sage and sea salt. Set the focaccia aside until it has risen and puffed around the toppings, about 30 minutes.
Step 5Place the focaccia on the center rack of the oven and bake until crisp and golden-brown, 30 to 40 minutes. The focaccia should have risen almost to the top of the pan, and the cheese will be crisp and browned.
Step 6Move the pan to the bottom of the oven and continue to bake until the bottom crust of the focaccia is deep golden and crisp to the touch, about 5 minutes more.
Step 7Remove the pan from the oven and remove the focaccia from the pan to a wire rack (use a fork to gently lift and slide the focaccia out of the pan, careful of any hot oil that may still be at the bottom of the pan). Brush the surface of the focaccia once more with olive oil. Set the focaccia aside to cool slightly -- or as long as you can resist it.
Step 8Transfer the focaccia to a cutting board. Halve the focaccia using a long knife, then cut one-half into 4 wedges (don't cut the other half until you are ready to eat it).