Heat the oven to 375°F. Spread the focaccia cubes on a rimmed baking sheet. Bake for 25 minutes or until lightly browned, stirring once halfway through the baking time. Transfer to a large bowl and let cool.
While the focaccia cubes are toasting, heat a 12-inch skillet over medium-high heat. Add the sausage and cook until well browned, stirring often to separate meat. Pour off any fat. Remove the sausage from the skillet and drain on paper towels.
Heat the butter in the skillet over medium heat. Add the mushrooms, celery, leeks and garlic and cook for 10 minutes or until tender and most of the liquid from the mushrooms has evaporated. Add the broth and heat to a boil.
Add the mushroom mixture, cooked sausage and parsley to the focaccia cubes and mix lightly. Spoon the stuffing mixture into a lightly greased 13x9x2-inch baking dish. Cover with foil.
Bake for 25 minutes. Uncover and stir the stuffing lightly. Bake for another 20 minutes, uncovered, or until the top is golden brown.
You can make this stuffing 1 day ahead and reheat. Prepare and bake the stuffing as shown above. Let cool to room temperature, then cover and refrigerate. To reheat, bake at 350°F. for 20 minutes covered with foil, then uncover and bake for another 10 minutes or until hot.
This nutritional information refers to each serving of the entire recipe and not just the products used as ingredients.