Combine oil, onion, garlic salt, basil, marjoram, thyme, and pepper in a heavy plastic bag; mix well. Add roast; coat well with marinade. Marinate in refrigerator 2 hours or overnight.
Preheat oven to 350 degrees F (175 degrees C). Line roasting pan with aluminum foil.
Combine kosher salt and water to form a thick paste. Pat 1 cup paste to a 1/2-inch thick rectangle in pan. Pat roast dry with paper towels; insert meat thermometer. Place roast on salt layer; pack remaining salt paste around meat to seal well. Bake for 60 to 70 minutes, or until thermometer registers 140 degrees. (Steam may cause crust to crack slightly.) Remove from oven; let stand 10 minutes. Remove and discard salt crust.