Add 4 cups of salt, rosemary, thyme, and whole black peppercorns to a medium bowl. Stir to combine.

Add a 1/2-inch layer of salt to the bottom of a non-reactive container. The smaller the container, the less salt you will need, but the salt must coat the entire surface area of the duck.

Place the duck breasts skin side up on top of the salt bed, ensuring the breasts aren't touching.

Completely cover the breast with the remaining salt mixture containing the herbs and pepper. Add more salt if needed to cover the duck breasts fully.

Cover the container with a lid or plastic wrap. Place them in the refrigerator for 24 to 48 hours.

Remove the breast from the salt. Give them a quick rinse with cold water.

Dry the breast thoroughly with paper towels or other lint-free cloth.

For optional added flavor, dust the meat in the ground pepper.

Roll the breasts with the cheesecloth until completely enclosed. Secure the cheesecloth with the butcher's twine so the breasts do not fall out. Alternatively, hand it by a meat hook towards the edge of the breasts.

Hang the breasts in your refrigerator with room for air circulation for 1 to 3 weeks. They should be somewhat firm to the touch. If they still seem a bit soft, let them hang for 3 or 4 more days and check again.

Slice at no more than 1/4 inch thickness with a sharp knife.