In small saucepan, bring broth to a boil. Place bulgur in medium bowl; pour boiling broth over bulgur. Let stand 20 minutes.
Meanwhile, heat oven to 350°F. Grease 12x8-inch (2-quart) baking dish. Melt margarine in large skillet over medium-high heat. Add onion; cook and stir 2 to 3 minutes or until onion is tender.
Reduce heat to medium; add squash, bell pepper, water, garlic salt and pepper. Cover; simmer 9 to 12 minutes or until squash is tender, stirring occasionally. Remove from heat; mash squash slightly. Stir in bulgur.
In small bowl, beat egg, milk and mustard until well blended; stir into squash mixture in skillet. Add 1/2 cup of the cornbread stuffing mix; mix well. Spoon mixture into greased baking dish; sprinkle with remaining 1/2 cup stuffing mix.
Bake at 350°F. for 30 to 35 minutes or until set.