Combine first 4 ingredients in a medium bowl; mix well. Shape mixture into 4 (1/2-inch-thick) patties.
Coat a large nonstick skillet with cooking spray; place over medium heat until hot. Add patties, and cook 4 to 5 minutes on each side or until done. Remove patties from skillet, and set aside.
Increase heat to medium-high; add mushrooms, onion, and thyme; cook, stirring constantly, 3 minutes or until vegetables are tender. Add sherry; cook 1 minute. Stir in gravy; return patties to skillet. Cook 2 minutes or until thoroughly heated. Garnish with thyme, if desired.