Rub salt and pepper into both sides of the steak. Put the steaks into a Ziploc bag that holds them comfortably. To the bag, add the onion, garlic, vinegar, and olive oil. Marinate the steaks at room temperature for up to 30 minutes.
Heat a large pan over medium-high heat. Cook the steaks, turning only once, to desired doneness. Remove from the grill and let rest 5 minutes. Slice the steak thinly against the grain before serving. Drizzle some of the chimichurri over the steaks.
For the Brava Sauce
Heat the olive oil in a pan and fry until brown.
Add the tomato and rosemary. Cook under a medium heat for around 10 minutes, stirring every now and then so that the sauce doesn’t stick to the pan.
Remove from the heat and let cool slightly. Blend with the immersion blender until you achieve a smooth sauce. Add salt and sugar.
Add the olive oil, little by little whilst you keep blending. You’ll slowly see how the sauce starts thickening until you achieve the texture of a mayonnaise.
For the Potatoes
Place the potatoes in a pot with enough olive oil to cover them. Cook for around 10 minutes under a low to medium heat. The idea is to slow cook the interior of the potatoes without burning them.
Increase the heat to its maximum and cook the potatoes until they start getting brown on the outside. Take them out of the pot and place them in a plate with a kitchen towel to absorb the oil excess. To finish the dish, toss the potatoes with the Brava sauce.
For the Chimichurri Sauce
In a small bowl, combine all the ingredients besides the olive oil. Once combined, slowly whisk in the olive oil. Serve with the steak!