Butter the bread with the softened butter and set aside.
In a skillet, melt the ½ cup butter and brown the mushrooms and onions over medium heat for 5 to 8 minutes, or until tender and the liquid has evaporated. season to taste with salt and pepper and set aside.
Brown the sausage, breaking it into bite size pieces.
In a greased lasagne pan, layer half of the bread, mushroom & onion mix, sausage, and cheese. repeat the layers, ending with the cheese. mix the eggs, milk, both mustards, nutmeg, 1 teaspoon salt, and 1/8 teaspoon pepper. pour over the above.
When ready to bake, sprinkle the parsley evenly over the top. bake for 1 hour at 350 degrees (fahrenheit).