Preheat your sous vide water bath to 135F.
Season liberally with salt. Place duck into bag and vacuum seal, or, into zip lock bag to use the water immersion method to seal the bag.
When the water is heated, drop your sealed duck breast into the water and cook for two hours. While the duck breast cooks, prepare the gratin.
When the timer goes off after two hours, remove the sealed duck breast from the water bath, discard bag and any accumulated liquid.
Place duck on a cutting board skin side up, and evenly score the skin (be careful to only cut through the skin). Salt the scored side well. Heat a skillet over medium-high heat and add 2 Tbsp butter. After the pan is hot and butter is melted, add the duck to the pan scored skin side down and cook until a golden, crisp skin forms, 2-3 minutes. Salt the other side of the duck
Flip duck breast and cook on the other side for 1 minute. Remove from pan, turn off heat and allow to rest while we make the orange sauce.
To the pan that we cooked the duck in, add white wine to deglaze for 2 minutes (your pan will still likely be quite hot, but if it isn't, go ahead and place the pan over low heat during this process). Then, add in juice from orange halves (do not discard yet!), honey, red wine vinegar and 1/2 tsp salt, plus more to taste. Allow sauce to gently mingle and thicken for 2-3 minutes until it coats the back of a spoon. Carefully add to a small bowl and set aside until ready to serve.
Slice duck breast(s) against the grain. Add slices to plates, drizzle with pan sauce and add zest from used orange halves on top.