Heat the oven to 400°F (200°C). Lightly grease a 9-by-13-inch baking dish.
If using two large breasts, slice each horizontally into two cutlets. Gently pound the thickest spots as needed so all four pieces are about ¾ inch thick. Season both sides with salt and pepper, then arrange in the baking dish in a single layer.
Stir the cream cheese, 1 tablespoon milk, garlic, and rosemary in a bowl until smooth. Add the remaining milk a little at a time if needed. Divide the mixture among the chicken pieces and spread it over the tops.
Mix the cranberry sauce and balsamic vinegar. Spoon it over the cream cheese, then sprinkle the cheddar on top. Toss the panko with the melted butter and scatter it over the cheddar.
Bake uncovered for about 22 to 30 minutes, until the thickest part of each piece reaches 165°F (74°C). Start checking at 22 minutes. If the crumbs brown early, loosely tent the dish with foil.
Rest for 5 minutes. Serve warm with a spoonful of the cranberry juices from the baking dish.