In a large skillet cook onion and garlic in hot pan sprayed with cooking spray over medium heat for 2 minutes. Add mushrooms and cook about 3 minutes more or until tender, stirring occasionally. Remove from heat. Add herbs.
In a greased 2-quart square baking dish, place half of the bread cubes. Top with mushroom mixture, ham and mozzarella.
In a mixing bowl combine beaten eggs or egg product, milk, sour cream, mustard, and pepper. Pour over layers in dish. Cover and chill in the refrigerator for 2 to 24 hours.
Bake, uncovered, in a 325 °F oven for 55 to 60 minutes or until a knife inserted near the center comes out clean. Let stand for 10 minutes.
To serve, cut into squares. If desired, garnish with fresh herb sprigs.