Cook, drain and crumble sausage into a medium size bowl. Add Spinach Souffle, onion 1/2 cup Swiss cheese and 1/3 cup milk. Stir well. In another medium size bowl combine 2 1/3 cups milk, Dijon mustard, eggs and black pepper. Whisk until well combined. Remove crusts from bread and cut into 1" pieces. In a greased 9x13" baking dish, place 1/2 of the bread pieces. Pour about 1/3 of the egg mixture over bread. Spread spinach mixture over bread, top with the rest of the bread pieces and then pour remaining egg mixture over all. Sprinkle with 1/2 cup Swiss Cheese. Refrigerate overnight. Preheat oven to 350°. Bake strata for 60-65 minutes or until set.