Chop 4 garlic cloves in a food processor until minced. Add the basil and pine nuts, and pulse a few times to combine. With the food processor running, slowly pour in the olive oil, stopping once or twice to scrape down the sides. Place the Parmesan cheese, lemon juice, and red pepper flakes into the food processor, and pulse until blended. Season to taste with salt and pepper. The pesto should be quite thick, but loose enough to drip off a spoon in clumps. Remove 1/3 cup of pesto; put the rest in an airtight container and refrigerate.
Rub the flat iron steaks with the reserved pesto, then cover with plastic wrap and refrigerate for 2 hours.
Preheat an outdoor grill or broiler for medium-high heat and lightly oil grate. Bring the steaks and pesto up to room temperature while the grill heats.
Grill or broil the steaks to desired degree of doneness on preheated grill, about 4 minutes per side for medium. Baste the steaks occasionally with a little of the pesto sauce. Once done, serve topped with the remaining pesto.