Picanha
With the tip of a knife, score the fat cap on the picanha in a 1-inch diamond pattern, cutting down to the flesh but not through it.
Finely grate the garlic cloves and combine them with the soy sauce, 2 tablespoons of the oil, lime juice, paprika, cumin, and pepper in a large bowl. Add the picanha and turn to coat with the marinade. Let stand at room temperature for 1 hour, or refrigerate up to 4 hours (Bring the roast to room temperature before cooking.)
Preheat the oven to 400°F.
Wipe off the excess marinade and season the meat with the salt. Heat the remaining 1 tablespoon oil in a large cast iron skillet over medium-high heat. Add the steak, fat cap down, and cook until browned, 2-3 minutes. Turn the steak over and cook 2 minutes longer.
Transfer the skillet to the oven and roast until an instant read thermometer inserted into the thickest part of the steak registers 120°F, about 45-50 minutes.
Remove from the oven, transfer to a cutting board and let rest, loosely tented with foil, 20 minutes before slicing. Serve with the charred scallion and garlic oil spooned over the slices.
Charred Scallion and Garlic Oil
Toss the scallions with 2 teaspoons of the oil. Heat a grill pan over medium-high heat until very hot. Add the scallions and cook, turning occasionally, until charred in spots, about 5-6 minutes. Transfer to a cutting board and cut into 3/4-inch pieces; transfer to a bowl.
Combine the remaining 1/2 cup oil, garlic, and red pepper flakes in a small skillet over medium heat. Cook until the garlic is lightly browned, about 4-5 minutes. Transfer the mixture to the bowl with the scallions and stir in the cilantro, lime zest, and salt. Serve warm or at room temperature. Can be made up to 2 days ahead and refrigerated.
