In large shallow dish, beat 3 tablespoons lime juice, 2 tablespoons olive oil, 1 teaspoon salt and 1/2 teaspoon pepper with whisk. Add flank steak; turn to coat. Cover and refrigerate 1 hour.
In 2-quart saucepan, heat water to boiling. Add rice and 1 teaspoon salt; cover, and reduce heat to simmering. Simmer 16 to 18 minutes or until water is absorbed and rice is just tender.
In small bowl, beat 1/4 cup cilantro, 3 tablespoons lime juice and 1 tablespoon olive oil with whisk. Stir into cooked rice; fluff with a fork.
In 10-inch skillet, heat 1 tablespoon oil over medium heat until hot. Add onion and serrano chile; cook and stir about 4 minutes or until vegetables are softened. Add taco seasoning mix, and stir to combine. Add beans, tomatoes and green chiles; heat to boiling. Reduce heat to low; simmer 10 minutes.
Set oven control to broil. Spray broiler pan rack with cooking spray. Place flank steak on rack in pan. Broil with top 3 to 4 inches from heat 6 minutes. Turn; broil 6 minutes longer for medium doneness. Transfer to cutting board; let stand 10 minutes. Cut across grain into thin slices, then into 1-inch pieces.
Divide rice among 6 serving bowls; top with bean mixture, steak and Toppings.
Guacamole makes another great topping for this addictive burrito.
Serrano chiles can add a little (or a lot) of heat to a dish. If you prefer your beans on the mild side, feel free to leave the chile out.
Beans can be made up to 2 days in advance. Reheat in a saucepan on low heat.