In large bowl, mix beef, panko, soup mix, and pepper with your hands until fully combined
Form mixture into four 4-inch-diameter patties, about ½ inch thick.
Using your knuckle, press down center of each patty to create slight indentation.
Melt 2 tablespoons butter in skillet that will hold all four burgers over medium-high heat.
Add patties and cook until well browned and meat registers 130 degrees, about 5 minutes per side.
Transfer to platter and tent with aluminum foil.
Reduce heat to medium, add mushrooms, if using and allow to cook for between 3 and 4 minutes.
Next add onion to skillet, and cook until lightly browned and beginning to soften, about another 4 minutes.
Stir in flour and cook for 1 minute.
Whisk in broth and bring to boil.
Cook until thickened, about 4 minutes.
Off heat, stir in remaining 2 tablespoons butter.
Season with salt and pepper to taste.
Spoon sauce over steaks and sprinkle with chives.