To prepare the dressing, whisk together the lemon zest and juice, the basil (or rosemary), mustard, garlic, and salt and pepper. Whisking constantly, add the lemon olive oil in a slow stream. Set aside.
Place the chicken on a cutting board and lay a sheet of wax paper over it. Using the flat side of a meat mallet, pound the chicken slightly so it is an even thickness.
Place the chicken in a shallow baking dish just large enough to hold it. Pour half of the dressing over the chicken and turn the breasts once or twice to coat. Cover the dish and refrigerate for at least 2 hours if possible, and up to 8 hours, turning at least once. Set aside remaining marinade until ready to serve.
If using a gas or charcoal grill, spray the grill with olive oil and prepare a medium-hot grill. If using a grill pan, spray it with olive oil and heat over medium-high heat.
Discard the marinade and grill the chicken until it is opaque in the center, 5 to 6 minutes per side.
Place the chicken on a platter and drizzle with the dressing. Serve.