Remove the ribs and seeds from the serrano pepper and mince. Combine 1 ½ tablespoons olive oil, balsamic vinegar, garlic, and half the serrano pepper in a shallow dish. Add the steak to the dish and flip to coat in the marinade. Season with salt and pepper. Let marinate for at least 15 minutes.
Halve and thinly slice half the onion, and set aside. Dice the remaining half onion.
In a small pot, heat ½ tablespoon olive oil over medium heat. Add the diced onion and cook for 2-3 minutes, until softened.
Add the rice to the onions and toss to coat, then add 1 ½ cups water and a large pinch of salt to the pot. Bring to a boil, cover, lower heat and simmer for 15 minutes, until rice is tender.
Peel and cut mango the mango into thin matchsticks and place in bowl. Zest and juice the lime, and add half the zest (reserving the rest) along will all of the lime juice to the bowl. Chop the cilantro stems and add to the bowl, reserving the leaves. Add the remaining serrano pepper and sliced red onion (to taste).
Cook the Steak:
Heat a large pan over medium-high heat. Add the steak and cook 4-5 minutes per side, until cooked to desired doneness. Set aside to rest for 5 minutes, then thinly slice against the grain.
Stir the cilantro leaves and remaining lime zest into the rice.