1 large sweet onion

3 cups buttermilk

2 cups minced fresh cilantro

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1 cup minced fresh parsley

2 garlic cloves, minced

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1/2 teaspoon plus 2 teaspoons salt, divided

1/4 teaspoon pepper

1/2 cup mayonnaise

1/2 cup plus 3 tablespoons olive oil, divided

1 beef flat iron steak or top sirloin steak (1-1/2 pounds)

1-1/2 cups all-purpose flour

3 tablespoons taco seasoning

Oil for deep-fat frying

12 slices Italian bread (1/2 inch thick)

Optional: Fresh arugula and sliced tomato

Cut onion into 1/4-in. slices; separate into rings. Place in a large bowl or shallow dish. Add buttermilk; turn to coat. Refrigerate at least 4 hours or overnight.

In a small bowl, combine cilantro, parsley, garlic, 1/2 teaspoon salt and pepper. Remove 2 tablespoons herb mixture; stir into mayonnaise. Refrigerate until serving. Stir 1/2 cup olive oil into remaining herb mixture. Reserve 2 tablespoons oil mixture for serving. Place remaining oil mixture in a large bowl or shallow dish. Add beef; turn to coat. Refrigerate at least 4 hours or overnight.

Drain beef, discarding marinade. Grill beef, covered, over medium heat or broil 4 in. from heat until meat reaches desired doneness, 7-9 minutes on each side (for medium-rare, a thermometer should read 135°; medium, 140°; medium-well, 145°).

Meanwhile, drain onion rings, discarding marinade. In a shallow dish, combine flour, taco seasoning and remaining 2 teaspoons salt. Roll onion rings in flour mixture. In an electric skillet or deep-fat fryer, heat 1 in. of oil to 375°. Fry onion rings, a few at a time, until golden brown, 1 to 1-1/2 minutes on each side. Drain on paper towels; keep warm.

Remove steak to a cutting board; brush with reserved 2 tablespoons oil mixture. Brush bread with remaining 3 tablespoons olive oil. Grill bread over medium heat until toasted, 30-60 seconds on each side.

Spread mayonnaise mixture over 1 side of each toast. Thinly slice steak. Top 6 slices of toast with steak, fried onions and, if desired, arugula and tomato slices. Top with remaining toast.