1 1/2 pounds lean ground beef
1 portobello mushroom, finely chopped
1/2 cup freshly grated Parmesan cheese
1/4 cup finely chopped onion
1/4 cup fine breadcrumbs
1/4 cup Worcestershire sauce
1/4 cup liquid egg substitute, well shaken
3 tablespoons chopped fresh parsley
1 teaspoon salt
1/2 teaspoon black pepper
6 hamburger rolls, split and toasted