Arrange turkey skin side down on a cutting board. Remove tenders and reserve for another use. Make a lengthwise cut about 3⁄4" deep down middle of each breast. Cover turkey with plastic wrap. Using the smooth side of a meat mallet, pound breast evenly to a 1 1⁄2" thickness. Season turkey with salt and pepper. Smear boudin over breast, leaving a 1" border around edges.
Beginning with one long side of the turkey breast, roll turkey into a cylinder so that the skin faces outward; set aside. Tie turkey crosswise at 1" intervals with eight 15" lengths of kitchen twine, then tie one 24" length of twine around length of breast to secure it. Trim excess twine with scissors. Place stuffed turkey on a plastic–wrapped baking sheet. Season turkey with salt and pepper; rub with oil, sage, and thyme and arrange garlic and lemon slices over turkey. Wrap with plastic wrap; chill overnight.
Heat oven to 350°. Unwrap turkey; remove garlic and lemon. Line a rimmed baking sheet with aluminum foil and set a rack inside baking sheet. Transfer turkey to rack and bake, basting with butter and turning turkey every 20 minutes, until an instant-read thermometer inserted into thickest part of the turkey reads 145°, 1–1 1⁄2 hours. Increase oven heat to 500° and continue cooking, turning once, until turkey is deep golden brown and an instant-read thermometer reads 150°, about 10 minutes more. Transfer turkey to a serving platter and let rest for 20 minutes. Remove kitchen twine. To serve, slice turkey crosswise into 1" pieces.