For the pesto: Combine the piquillo peppers, macadamia nuts, salt and olive oil in the bowl of a food processor. Process until smooth, scraping down the sides with a rubber spatula as needed. Spoon the pesto into a bowl and set aside until ready to use.
For the steak: Remove the flat iron steak from the refrigerator 30 minutes prior to cooking. Heat a large skillet over medium-high heat. Pat the steak dry with paper towels and sprinkle all over with the salt. Add the oil to the skillet, swirling to coat the bottom of the pan. Carefully add the steak to the pan and place a foil-wrapped brick or a small cast-iron pan on top of the steak to weight it down. Cook for 3 to 4 minutes on the first side. Remove the weight and, using tongs, flip the steak. Cook for an additional 3 to 4 minutes for medium-rare to medium doneness. Remove the steak from the pan and let rest for 10 minutes before slicing against the grain. Serve with the pesto.