Combine dressing ingredients in small bowl. Reserve 1/4 cup dressing for salad. Brush remaining dressing on onion slices.
Press 1 tablespoon seasoning blend evenly onto beef steak. Place steak in center of grid over medium, ash-covered coals; arrange onion slices around steak. Grill steak, covered, 7 to 11 minutes (over medium heat on preheated gas grill, covered, 8 to 13 minutes) for medium rare (145°F) to medium (160°F) doneness, turning occasionally.
Alternatively, prepare on stovetop by heating either a large nonstick skillet or grill-pan over medium heat until hot. Place steak in skillet/grill-pan; cook 7 to 10 minutes for medium rare (145°F) to medium (160°F) doneness, turning occasionally.
Grill onion 10 to 12 minutes (gas grill times remain the same) or until tender, turning occasionally (can sauté onion on skillet/grill-pan if cooking indoors).
Carve beef into slices (always slice against the grain). Divide spinach evenly among four plates. Top with steak slices, tomatoes, onions and eggs. Drizzle evenly with reserved 1/4 cup dressing.