Combine oil, onion and seasonings in heavy plastic bag; mix well.
Add roast; coat well with marinade and refrigerate 2 hours to overnight.
Preheat oven to 350°F Line roasting pan with aluminum foil.
Combine Kosher salt and water to form a thick paste. Pat 1 cup of paste into a 1/2 inch thick rectangle in foiled lined pan. Place roast on salt layer; pack remaining salt paste around meat to seal well.
Bake at 350°F 60 - 70 minutes or until thermometer registers 135°F (Steam may cause salt crust to crack slightly during roasting.).
Remove from oven; let stand 15 minutes. Remove and discard salt crust. Carve thinly.