Very slowly cooking a steak at low temperature until it reaches the desired internal temperature and then searing it quickly over high heat to get a crust is known as reverse-searing, and it takes a lot of the guesswork out of cooking a steak perfectly. In this recipe, I use the sirloin cap—the triangular muscle that sits on the top sirloin—called a coulotte in France and a picanha steak in Brazil. Whatever you call it, this cut has a natural fat cap, which I leave on while cooking so the rendered fat bastes the steak. If you prefer, you can remove the fat or ask your butcher to do it for you.
Note: Start this at least 12 hours before you plan to eat.
Place the garlic, thyme, vinegar, oil, and a pinch of salt in a small bowl and mix together.
Using the point of a sharp knife, score the fat side of the sirloin cap in a crosshatch pattern. Place the steak in a baking dish, and season both sides liberally with salt and pepper. Rub the marinade all over the steak, and refrigerate, uncovered and fat cap facing up, for at least 8 hours.
An hour and a half before you want to eat, preheat the oven to 275°F. Take the steak out of the fridge and bring to room temperature. Place the steak in the oven and cook until an internal thermometer inserted into the middle of the steak reads the desired temperature. Take the steak out of the oven and set aside.
Place a large, heavy-bottomed frying pan over high heat. Discard the garlic and thyme, and then sear the sirloin cap, fat side down, in the hot pan. Turn the steak over when golden, and sear the other side. The whole process should take about 1 minute per side. Remove from the pan and set on a cutting board to rest for 5 minutes.
To make the chimichurri sauce, place the cilantro in a blender and purée with the sugar, oil, lime juice, and vinegar. Pour the mixture into a serving bowl and stir in the red onion, garlic, and lime zest. Season with salt and pepper.
To serve, slice the steak against the grain and arrange on a serving platter with the chimichurri alongside.