Sprinkle the chicken with a generous amount of salt and pepper on both sides.
In a small container or a bowl, shake up or whisk together the olive oil, herbs, mustard, and garlic.
You can either place the chicken in a large zipper top baggie or two, and add the herbed oil, and squish it all around to coat well, or you can layer up the chicken with the herbed oil in a large container with a lid. Either way, seal it up once the chicken is all coated, and refrigerate from 4 to 12 hours.
Preheat a grill to medium, and grill the chicken breasts for about 4 to 6 minutes per side, depending on thickness, until it is almost cooked through (there should be only a slight hint of pinkness remaining in the center).
Transfer the chicken to a cutting board, and let sit for 4 or 5 minutes, then slice, and serve however you wish. This is also great at room temperature.
You can serve the breasts attractively sliced up as a main course, dice them, and use them in a pasta or rice salad, serve slices over any number of green or vegetable salads, toss them into grilled quesadillas, and so on. Make more than you need because they are one of the most versatile summer leftovers I can think of.