For the pesto: To the bowl of a food processor, add the almonds, olive oil, parsley, sage, Parmesan, garlic, lemon juice and pepper flakes. Process until smooth.
For the steak: Heat a cast-iron skillet over medium-high heat until very hot. Season the steaks on both sides with salt and pepper. Add the oil to the pan and, when it shimmers, add the steaks. Cook for 4 to 5 minutes on each side for medium rare. Remove to plates and let rest for 5 minutes.
Top the steaks with a dollop of pesto and serve.
Remaining pesto can be stored in an airtight container in the refrigerator for up to 4 days.