Place each breast between two sheets of plastic wrap. Pound to an even 3/4-inch thickness using a meat mallet or rolling pin. This takes about 90 seconds per breast.
Whisk together olive oil, lemon zest, Dijon mustard, honey (if using), garlic, oregano, thyme, salt, and pepper. Add lemon juice only if marinating 2 hours or less. The marinade will look thick and golden.
Add chicken to marinade, coat well, press out the air, seal, and refrigerate at least 30 minutes. Two hours is ideal for breasts. Up to 8 hours is fine for thighs or drumsticks.
Heat grill to medium-high (around 400-425F). Clean the grates, then brush with oil just before adding the chicken.
Lay each piece on the grill and leave completely alone for 6-7 minutes. Flip when the sear has formed and the chicken releases naturally. Grill second side 5-7 minutes. Place lemon halves cut-side down alongside for 4 minutes.
Use an instant-read thermometer to check the thickest part. Pull chicken at 160F (71C). Carryover cooking brings it to the safe 165F (74C) during rest.
Rest 5 minutes loosely tented with foil. Squeeze the charred lemon halves over the chicken. Garnish with fresh herbs and a pinch of flaky salt. Serve immediately.