Heat oven to 425 degrees F. Blend butter, lemon zest, dried thyme, 3/4 teaspoon salt and 1/4 teaspoon black pepper. Rub half under skin of turkey breast and other half on outside of skin.
Place turkey on rack in roasting pan and pour in 1 cup water.
Roast turkey at 425 degrees F for 30 minutes. Reduce oven to 325 degrees F; roast 50 minutes or until breast registers 160 degrees F on an instant-read thermometer. Rest 20 minutes.
Pulse carrot, celery stalk and small onion, halved, in a food processor until finely chopped.
Melt unsalted butter over medium-high heat. Add veggies; cook 7 minutes. Add flour; cook, stirring, 4 minutes. Lower heat to medium. Whisk in low-sodium chicken broth, lemon juice, 1/2 teaspoon salt and 1/8 teaspoon pepper; simmer for 13 minutes; strain well.