Pat the turkey breast dry with a paper towel. Let rest at room temperature for 30 minutes while you prepare other ingredients.
Preheat oven to 220 degrees C (425 degree F).
Prepare a roasting pan. Spread out the carrot, leek, white onion and garlic. Add chicken stock.
Combine all the ingredients for the hoisin butter sauce into a food processor. Puree until it becomes a fine paste. If you don’t have a food processor, finely chop garlic, green onion, shallot, and ginger. Mix everything together by using a mortar and pestle to form a paste.
Separate the skin from the turkey breast by gently sliding your finger between the meat and the skin. Try not to tear the skin. Leave the edges attached, so it shape a pocket to hold the butter. If you cannot stuff all the butter in, save the rest to use later (as chicken marinade or sauce).
Pour the paste in between the skin and the meat. Press your finger on top of the skin to distribute the paste evenly underneath. Sprinkle sea salt on both sides of the turkey breast.
Place the turkey breast onto the roasting pan with vegetables. Bake until the internal temperature reaches 60 degrees C (140 F), 50 minutes to 1 hour. The residue heat will keep cooking the turkey while resting.
Once the turkey is cooked through, remove it from the roasting pan and place onto a plate. Loosely cover with foil and let rest for 15 to 30 minutes.
Strain the liquid from the roasting pan into a big measuring cup. Discard the solid ingredients. Skin off the fat with a ladle and discard, if needed. Reserve the juice. Add more stock, so you should have 2 cups of stock in total.
Add butter into a saucepan and cook over medium heat until melted. Turn to medium low heat. Whisk in flour. Stir and cook until golden brown.
Gradually add turkey drippings and stock, keep stirring, until it forms a silky paste. Cook until boiling before adding more turkey drippings, so the gravy won’t separate. Once the turkey drippings are added, bring to a light boil. Let simmer and cook until it reduces to 3 cups, after 10 to 15 minutes. Taste the gravy and season accordingly by adding salt and pepper to taste.
Serve immediately in a gravy boat with turkey.
You can also use this recipe to cook a whole turkey breast. Note that in this case you should adjust the oven rack to a lower position, so the whole breast will be right in the middle of the oven.
In this recipe I made extra gravy so you can serve it with mashed potatoes.