In small saucepan, combine broccoli and 1/2 cup water. Bring to a boil. Reduce heat; cover and simmer 2 to 3 minutes or until broccoli is crisp-tender. Drain well.
Meanwhile, place toasted bread slices on ungreased cookie sheet. In small bowl, combine mayonnaise, horseradish and mustard; mix well. Spread mixture on each bread slice.
Top each bread slice with mushrooms, cooked broccoli, tomato slices and onion rings. Top each with 2 cheese slices.
Broil 4 to 6 inches from heat for 2 to 4 minutes or until cheese is melted. Serve immediately.