Preheat oven to 425º. Scatter chopped onion, lemon slices and fresh herbs in bottom of pan, under the rack. Roast for 30 minutes and then turn oven temperature down to 350º. Roast until desired doneness, 135º for medium-rare, 145º for medium; about 1½ hours more. Add about 2 cups chicken stock to the pan after 1 hour and add some more as needed after about another ½ hour. Pull the roast out of oven 5 degrees shy of goal temperature, transfer to cutting board or platter, tent with foil and let rest 20 to 30 minutes.