In a large bowl stir together the spinach souffle, rice, milk, 1/2 cup Swiss cheese, onion, mustard and salt. Transfer mixture to a 9x13 inch baking dish, and cover with aluminum foil or a lid.
Bake for 25 minutes in the preheated oven.
Meanwhile, in a large nonstick skillet, saute the chicken breasts in oil until both sides are golden brown and the juices run clear. Arrange the chicken on top of the spinach mixture.
In a small bowl, combine the bread crumbs, butter or margarine and remaining 1/2 cup Swiss cheese; sprinkle over the chicken. Bake uncovered for an additional 25 minutes, or until the spinach mixture is set, the chicken is cooked through and juices run clear.
