Spice-Rubbed Steak Salad with Creamy Red Wine Vinaigrette

Steak:

One 1 1/2- to 2-pound skirt steak

4 teaspoons kosher salt 

4 teaspoons sugar 

2 teaspoons freshly ground black pepper 

1/2 teaspoon cayenne pepper 

1 tablespoon vegetable oil 

1/2 Vidalia or sweet onion, cut crosswise into 1/2-inch rings 

Creamy Red Wine Vinaigrette:

3 tablespoons red wine vinegar

2 tablespoons mayonnaise 

1 tablespoon honey 

1/4 teaspoon red pepper flakes 

2 cloves garlic, minced or pressed 

3 tablespoons olive oil 

1/4 teaspoon kosher salt, plus more if needed

1/4 teaspoon freshly ground black pepper 

Salad:

2 heads iceberg lettuce, cut into bite-size pieces

2 ounces blue cheese, crumbled into chunks 

1 cup grape tomatoes, halved