Combine mustard, honey, dried oregano, and lemon juice, if using, to make sauce.
To ground beef, add 2 tbls. of the above mustard sauce, oregano, onions, salt and pepper. Mix together lightly, but make sure it is fully incorporated. Form into 4 burgers. Grill or broil 4 to 5 minutes on each side for medium. When the burgers are almost cooked, place a slice of cheese on each one. Place on bottom of bun and top with lettuce and tomato. Spoon the rest of the honey mustard sauce over the top and place top bun over all. Enjoy!