Prepare Horseradish Sauce:.
In a small bowl, stir together mayonnaise, horseradish, lime juice and oregano. Cover and chill.
In a small bowl, combine oregano, coriander, salt and pepper: set aside.
In a 12" nonstick or cast iron skillet, melt butter over medium low heat. Add onions, garlic, and 2 teaspoons of spice mixture(half). Cook, covered, for 13-15 minutes until onions are tender, stirring occasionally. Uncover; cook and stir over medium high heat for 5-8 minutes more or until onions are golden. Remove from skillet; set aside. Wipe the skillet clean.
If using flank steak, score meat on both sides by making shallow cuts at 1" intervals diagonally across steak in a diamond pattern. Rub steak on both sides with the remaining half of the spice mixture.
Heat the same skillet over medium high heat until very hot. Add steak and cook, covered, for 12-16 minutes or until meat is medium rare(145*F.), turning once. Remove meat from skillet to cutting board; cover with foil. Let steak stand 10 minutes.
Thinly slice steak across the grain.
To serve, arrange lettuce leaves and steak on bottoms of rolls. Top with onions, dollop with horseradish sauce. Add roll tops. Enjoy!