1⁄2 cup chopped mixed fresh herbs (such as thyme, rosemary, marjoram, parsley)
1 alder grilling plank
4 rosemary sprigs (optional)
Wrap each steak with a slice of bacon and secure with a wooden toothpick; place in a baking dish. Wipe mushrooms with a dry cloth (do not get them wet). Discard stems. Place caps in baking dish. In a small bowl, stir together 1 cup port wine, 1 cup beef broth, au jus gravy mix, vinegar, garlic, and chopped herbs. Pour over steaks and mushrooms in baking dish. Cover with plastic wrap. Marinate in refrigerator for 2 to 4 hours.
One hour before grilling, soak plank in remaining 1 cup each port wine and beef broth (leave in mixture until ready to grill). Thirty minutes before grilling, remove steaks and mushrooms from refrigerator to come to room temperature.
Meanwhile, set up grill for direct cooking over medium-high heat. Oil grates when ready to start grilling.
Remove plank from wine-broth mixture, place on grill, close lid, and grill for 3 minutes. Plank may warp; turn plank every minute until it flattens out.
Remove steaks and mushrooms from marinade (discard marinade). Place steaks on plank, close lid, and grill for 10 minutes. Turn steaks and place a mushroom cap on top of each steak. Close lid and grill for 4 to 5 minutes more for medium-rare (145°F). Remove steaks from grill, cover, and let stand for 5 minutes. Serve with the mushroom caps either on top of or underneath the steaks. Garnish each with a rosemary sprig, if desired.