Place chicken breasts in a 9x13 inch baking dish. Put several slices of onion on top.
In a medium saucepan over medium heat, stir in ketchup, vinegar, butter, garlic, rosemary, salt, and mustard. Bring to a boil, then pour sauce over chicken.
Bake in a preheated oven for 30 minutes. Turn chicken breasts over, and continue baking until done, about 20 to 30 minutes.