Cover spinach and microwave 2 to 3 minutes until tender. Drain.

Preheat large skillet on medium. Cut steaks into 4 portions. Sprinkle both sides with salt and pepper. Place butter in pan then add steaks and onions. Cook for 5 to 6 minutes on each side.

Stir feta, bread crumbs and pesto in with spinach. Top steaks with spinach mixture in last two minutes of cooking. Brown topping in broiler or by turning steaks over for 1 to 2 minutes.