Take the duck breasts out of the refrigerator and leave at room temperature for approximately 15 minutes before cooking.
Pat dry with a paper towel. Remove silver skin if desired. Trim excess fat from around the duck breasts. Make incisions on the skin in a crosswise pattern, without cutting into the meat. Season both sides with salt and pepper.
Preheat a frying pan to medium-high heat, grill the breasts with the skin side down for 2 minutes.
Reduce the heat to medium. Crush three garlic cloves in their skins and add to the frying pan, along with two sprigs of rosemary.
Cook for another 5 minutes skin side down, occasionally spooning the rendered fat over the duck breast.
Turn and cook flesh side down for 5 minutes or until it reaches an internal temperature of 58°C (137°F), occasionally spooning rendered fat over the breast. The breast should be medium firm to the touch and still pink inside.
Remove the duck breast from the pan and place on a plate. Let stand for 5 minutes, loosely covered with a piece of aluminum foil.
Slice slanted or serve whole. Season with “fleur de sel” to taste.