For the steak: Place the steak on a large plate. Mix together the salt, sugar, pepper and cayenne in a small bowl. Rub the spice mix all over the steak.
Heat a grill or grill pan to medium-high heat. Brush with the vegetable oil.
Add the steak and onions to the grill and cook, without disturbing, 2 to 3 minutes; flip and cook the other side for another 2 to 3 minutes. Remove the steak to a cutting board to rest before carving, 6 to 8 minutes. Transfer the onions to a cutting board and roughly chop.
For the creamy red wine vinaigrette: Whisk together the vinegar, mayonnaise, honey, red pepper flakes and garlic in a large bowl. Slowly whisk in the olive oil until combined and smooth. Season with salt and pepper.
For the salad: Add the lettuce, cheese, tomatoes and grilled onions to the bowl with the dressing and toss to coat. Serve with the sliced steak.