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Slice top off heads of garlic. Brush with 2 tsp. (10 ml) of oil and wrap in aluminum foil. Bake in 350°F (180°C) oven for 1 hour or until tender. Remove garlic from oven and let cool.
In the meantime, slice 1 onion into rings. Spread rings across the bottom of a roasting pan and place frozen turkey on top. Pour 1 cup (250 ml) of chicken broth in the roasting pan. Cover and bake in 350°F (180°C) oven for 3 hours 30 minutes.
In a small bowl, squeeze out pulp of 1 garlic head. Add the honey and mustard and combine. Add salt and pepper and set aside.
Dice the remaining onion. Heat the remaining oil in a saucepan over medium heat. Add the diced vegetables and cook, stirring occasionally, for 5 minutes or until onion is translucent. Add barley, 3 bay leaves, parsley and pulp of 1 head of garlic into the saucepan. Continue cooking for 5 minutes, stirring frequently. Add salt and pepper. Add 2 cups (500 ml) of the remaining chicken broth and 1 cup (250 ml) of chicken stock and bring to a boil. Turn off heat and set aside.
Remove roasting pan from oven. Discard onion and skim the fat off the pan drippings. Place reserved barley mixture in roasting pan around turkey. Brush turkey with reserved garlic and honey mixture.
Cover roasting pan and return to oven for 1 hour or until meat thermometer reads 170°F (80°C). Remove cover and bake for another 30 minutes or until thermometer reads 185°F (85°C).
In the meantime, add pulp of remaining head of garlic into a blender. Add remaining broth and stock and mix until smooth. Pour mixture into a small saucepan set over medium heat. Combine butter and flour in a small bowl. Whisk the butter mixture into the prepared liquid. Add brown sugar and remaining bay leaf and continue cooking, stirring frequently, until sauce thickens (add more chicken broth as needed). Keep warm.
Remove turkey from oven and let stand 15 minutes. Serve with sauce and barley mixture.