Prepare Horseradish Sauce; cover and chill. In a small bowl combine oregano, coriander, salt, and pepper; set aside. In a 12-inch nonstick or cast iron skillet, melt butter over medium-low heat. Add onions, garlic, and half (about 2 teaspoons) of the spice mixture. Cook, covered, for 13 to 15 minutes until onions are tender, stirring occasionally. Uncover; cook and stir over medium-high heat for 5 to 8 minutes more or until onions are golden. Remove from skillet; set aside. Wipe skillet clean.
If using flank steak, score meat on both sides by making shallow cuts at 1-inch 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. Cook, uncovered, for 12 to 16 minutes or until meat is medium-rare (145 degree F), turning once. Remove meat from skillet to cutting board; cover with foil and let 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. Makes 6 sandwiches.
In a small bowl stir together mayonnaise, prepared horseradish, lime juice, and oregano.