Melt butter in a saute pan over medium heat. Cook mushrooms and onions for two minutes or until just slightly tender. Remove with a slotted spoon. Dip eight b read slices into pan to butter one side. Place bread, butter side down, in an o blong casserole (13 x 9 x 5 cm). Layer half of the mushrooms and onions, ba con, celery, and chilies on top of the bread and finish with half of the cheese . Season with salt, pepper, and one tablespoon of dill. Repeat the bread layer and the remaining half of ingredients as above, but add tomato slices before ch eese.
Thoroughly mix eggs, cream, and mustard and pour over casserole. Cover with wax ed paper and refrigerate overnight.
Before baking allow casserole to stand at least 30 minutes at room temperature. Preheat oven to 180ºC . Bake casserole uncovered for 45 minutes or until souffle like consistency is achieved. Remove from oven and cool five minutes before sli cing. Leftovers may be frozen.