To prepare Caramelized Three-Onion Sauce, heat oil in large non-stick skillet over medium heat until hot. Add red, yellow, and green onions, cook and stir for 8 to 10 minutes or until onions are tender and beginning to brown.
Reduce heat to low; cook 12 minutes or until onions are caramelized, stirring occasionally. Increase heat to medium high. Stir in vinegar, cook 4 minutes or until reduced by half stirring occasionally. Stir in sugar; season with salt and pepper, as desired. Reduce heat to low; cook 5 minutes or until liquid evaporates and onion mixture thickens.
Combine all ingredients for the Ancho Steak Seasoning in a small bowl; store in airtight container. Shake well before using.
Press Ancho Steak Seasoning evenly onto beef roast. Place roast on grid over medium, ash-covered coals. (We used our gas grill with no problems.)
Grill, covered, 25 to 35 minutes for medium-rare to medium doneness, turning occasionally. Remove roast when instant-read thermometer inserted into center of thickest part of beef registers 140°F for medium-rare, 155°F for medium. Let stand for 10 minutes. (Temperature will continue to rise about 5°F to reach 145°F for medium-rare, 160°F for medium.)
Carve roast across the grain into thin slices and serve with Caramelized Onions.