Combine Ground Beef, Egg, Panko Bread Crumbs, Onion, And 1/4 Cup Cream Of Mushroom Soup In A Large Bowl. Divide Beef Mixture Evenly Into 6 Portions; Shape Each Portion Into A 1/2-Inch Thick Patty. Heat A Large Non-Stick Skillet Over Medium-Heat. Cook The Burgers Until Browned, 3 To 4 Minutes On Each Side. Whisk Together The Remaining Cream Of Mushroom Soup And Beef Broth In A Bowl Until Smooth. Pour Mixture Over Patties, Then Reduce Heat To Low. Simmer Until The Patties Are No Longer Pink In The Center, About 20 Minutes.