Preheat outdoor grill or broiler and cut Vidalia onions into in 8 (1/2 inch thick) slices. Prepare the dressing by combining orange juice, oil, vinegar, honey, salt and black pepper in a small bowl.
Place onion slices on a grill or broiler pan. Grill until slightly softened and golden brown, about 3 minutes; turn and cook until tender, about 3 minutes longer. Spoon half of the dressing over the onions and remove from grill.
Serve on a bed of spinach leaves topped with chopped tomatoes and drizzled with remaining dressing.