Cut garlic cloves into thin (about 1/8-inch thick) slices.
With small sharp knife, cut 1-inch-deep slit into meat. Do not remove knife. Angle knife slightly to open slit; slide in garlic slice. Repeat, inserting garlic all over beef.
Combine steak seasoning, 1/2 t. salt and pepper; rub over beef. Place on rack in roastin gpan; roast 20 minutes.
Toss mushrooms and onion with oil and remaining salt; add to pan. Reduce heat to 300 degrees. Roast until thermometer inserted into thickest part registers 140 degrees for medium-rare, 20-30 minutes. Let stand 15 minutes. Serve with vegetables.