Add broccoli rabe and 1 tablespoon water. Cover; cook 8 minutes, until crisp-tender, stirring occasionally. Set aside and keep warm.
Meanwhile, sprinkle salt and freshly ground black pepper, both to taste, on steaks.
Heat large nonstick skillet coated with cooking spray over medium-high heat until hot.
Add steaks; cook 3 minutes per side for medium-rare, or longer for desired doneness. Using a meat thermometer, check that meat has reached an internal temperature of 14
Transfer steaks to a serving plate; keep warm.
Add mushrooms to the skillet. Cover; cook over medium-high heat, 3 minutes.
Add port. Simmer, uncovered, 3 minutes, until sauce thickens slightly.
Serve mushrooms over steaks; top with thyme. Season broccoli rabe with salt and freshly ground black pepper, both to taste.