Pat duck breast dry. Do not season (or go very light on salt). Optional: Add a few pats of butter and fresh herbs.
Place in sous vide bag. Put in 54° C / 130° F water for 1-4 hours.
Remove duck from bag and pat dry. Season with salt & pepper (can go heavier on the salt on skin side).
Place duck breast skin side down in cold heavy bottomed pan or cast iron skillet. Set to medium-high heat. When pan is to temperature, cook for 2 minutes, allowing fat to render out and skin to get crispy. Reduce heat to medium and cook another ~5 minutes, pressing duck to ensure good contact between skin & pan until golden and crisp.
Flip and cook other side until barely colored, about 30 seconds-1 min.
Allow to rest 5 minutes off the heat. Slice breast into 1/2″ strips and serve.