Prepare the chicken: Season chicken breasts with salt and pepper to taste. Pound the thickest part of the breast, so that the whole breast is of almost even thickness (it will cook more evenly). You can also cut chicken breasts in half lengthwise (with your knife blade parallel to the cutting board) - it will cook faster this way.
Sear the chicken: Heat the oil over high heat in a big frying pan. Once hot, add the chicken pieces, cook without stirring for 2 minutes or until browned, then cook on the other side until browned. Transfer the chicken to a plate. The cooking time will depend on the pan type and how crowded the chicken is. It doesn't need to be cooked through in the middle, just golden brown on both sides.
Make the sauce: Reduce the heat to medium and add finely chopped garlic to the pan. Cook, stirring for 30 seconds, or until fragrant. Add all the other ingredients for the sauce: cranberries, rosemary, chicken broth, orange juice and zest, vinegar, honey, mustard. Season lightly with salt and pepper (only lightly, the sauce will reduce its volume and can become too salty).
Bring to a boil, then add the chicken pieces back to the pan. Cook over medium-low heat until the chicken is cooked through (flip the chicken breast halfway through). Cooking time depends on how big (thick) the chicken breasts are and how long have you seared them. Small chicken breasts (or cut in half lengthwise breasts) will need just a couple of minutes, bigger chicken breasts will need about 10 minutes.
Transfer the chicken breasts to plates. Remove the rosemary twigs from the sauce, season the sauce with salt and pepper, if necessary. If the sauce is too sour for your liking, add more honey, if too sweet just add a little more vinegar or lemon juice. It should have a nice sweet and sour balance of flavors, but it needs to be rather on the sweet side because the cranberries are rather tart (you need to try the sauce with the cranberries).
Serve and enjoy!