In medium bowl, mix beef, bread crumbs, milk, oregano, fennel, garlic powder and pepper flakes. Shape mixture into 4 patties, 1/2 inch thick.
Heat 8-inch nonstick skillet over medium heat. Add onion, bell pepper and water; cover and cook 5 minutes, stirring occasionally, until vegetables are crisp-tender. Remove vegetables from skillet; cover to keep warm.
Wipe skillet clean; heat again over medium heat. Add patties; cook 10 to 12 minutes, turning once, until thermometer inserted in center of patties reads 160°F. Serve patties and vegetables in buns. Top with mustard.