It is important to thin the chicken breasts by cutting them in half. Otherwise, it will take longer to cook through, and you may have burned/charred honey mustard on the outside of your chicken.
You will likely have some burnt honey on your pan despite using thinner cuts of chicken for faster cooking time. This has happened every time I have made this dish, but don't worry - it doesn't burn on the chicken itself or taste charred at all. The chicken turns out with a lovely golden brown every time. I usually have to wash/scrape the burnt honey out of my pan between chicken batches, however.
The whole grain mustard pops and splatters a lot while cooking, so it makes a bit of a mess. I have not tried this on the grill, but I imagine it would work out great, with less mess to clean up afterwards!