Preheat the oven to 325F and line a baking sheet with parchment paper. Grease the paper.
Follow the directions for making the focaccia dough, but be sure to omit the rosemary and the water. Turn the dough out onto the prepared pan and use your hands to press out into a 10x14 inch rectangle.
Bake 15 minutes, until just firm to the touch. Remove and let cool while preparing the topping. Raise the oven temperature to 375F.
Add 2 tablespoons of the oil to a medium skillet over medium heat. Once hot, add the mushrooms and sprinkle lightly with salt and pepper. Cook 5 minutes, until the mushrooms are beginning to brown.
Add the garlic and chopped sage, and cook another 2 minutes, until fragrant. Remove from heat.
Sprinkle the cheese evenly over the crust, leaving a ½ inch border. Layer on the cooked mushrooms and bake 10 to 15 minutes, until the crust is golden and the cheese is melted.
Meanwhile, heat the remaing oil in the skillet over medium heat. Once hot, add the whole sage leaves and cook until crispy, 1 to 2 minutes.
Remove the bread from the oven and place the sage leaves in rows on top, one for each slice. Sprinkle lightly with the sea salt and cut into slices.