1/4 cup maple syrup
2 tablespoons unsalted butter
3 large rosemary sprigs
1 tablespoon kosher salt, plus more as needed
3 pounds boneless, skinless turkey breast, cut into halves, skin reserved if desired (see Note)
Crisp turkey skin, optional (see Note)
In a small saucepan, simmer maple syrup, butter, rosemary and salt until the butter melts and the maple syrup thickens slightly, about 5 minutes. Let cool.
Place the turkey in a large bowl and pour the cooled maple mixture over it, turning to coat pieces with syrup. Cover and refrigerate overnight.
Using a sous-vide machine, heat a large pot or heatproof container of water to 145 degrees. Transfer turkey to a reusable silicone sous-vide bag or resealable plastic bag, laying breasts end-to-end so the thin side of one is against the thick side of the other. Slowly lower the bag into the water, allowing any air to escape. Weigh down the bags; an upside-down metal steamer basket or a metal bowl work well for this, topped with a metal spoon or tongs for extra weight, if needed. Cook until the turkey reaches 145 degrees when checked in the center with a meat thermometer, 2 1/2 to 3 hours.
Carve the breast, and serve with crisp turkey skin, if you like.
If want to make the crisp turkey skin, ask your butcher to save you the skin when your order the breast (or be sure to save it if you’re trimming your own turkey breast). Scrape any visible lumps of fat off the skin. Heat oven to 325 degrees and line a rimmed baking sheet with parchment paper. Pat skin dry with paper towels, then spread it out flat on prepared baking sheet. Sprinkle lightly with salt, and bake until skin is golden and crisp, anywhere from 45 minutes to 1 1/2 hours.