Saute mushrooms with pepper in hot low-fat margerine until tender and lightly browned, about 5 minutes.
Preheat oven to 375 degrees.
Slice croissants in half, lengthwise. Spread bottom half with 1 teaspoon dijon mustard; top with roast chicken, mushrooms, tomato, onion and swiss cheese slices.
Replace tops of croissants. Place on cookie sheet; cover loosely with foil. Bake for 15 minutes or until cheese is just melted and sandwich is heated through.