Marjoram is added toward the end of cooking to retain its flavor.
Rub garlic, 1/4 teaspoon salt, and 1/2 teaspoon pepper evenly over both sides of steaks; brush both sides with oil. Cover and let stand 10 minutes.
Cook mushrooms in butter in a large skillet over medium-high heat, stirring constantly, 1 minute. Stir in remaining 1/4 teaspoon salt and 1/4 teaspoon pepper, marjoram, and sherry; bring to a boil. Reduce heat, and simmer, uncovered, stirring occasionally, until mushrooms are tender and liquid is slightly reduced (about 5 to 7 minutes). Remove from heat; cover and keep warm.
Coat grill rack with cooking spray; place on grill over medium-hot coals (350 to 400 degrees). Place steaks on rack, and cook 4 minutes on each side or to desired degree of doneness. Serve mushrooms over steaks.