Set a large ziplock bag upright and add dijon mustard, honey, tamari and yogurt. Whisk to mostly combine.
Slice chicken breasts into three pieces each. Spread a large piece of saran wrap (approximately 2 feet long) on the counter and add chicken breast pieces.
Top with another piece of saran the same size. Use a meat mallet or other blunt heavy object (get creative, lol) to thin the breasts to approximately 3/4" cutlets. Add cutlets to your ziplock bag and tightly seal, removing excess air.
Shake bag to evenly distribute marinade over the cutlets. Wipe a soapy sponge over the ziplock to make sure it doesn't have any chicken bacteria, then refrigerate for at least three hours and up to 24 hours.
Preheat oven to 400 degrees F and line a large baking sheet with nonstick parchment paper. Heat a nonstick grill pan or cast-iron pan to high heat (turn on any air vent you have).
If you don't have a nonstick surface, coat with 1 teaspoon oil. Test to see if surface is hot enough by adding a splash of water. If it sizzles, it's ready.
Use tongs to pull chicken pieces out of the ziplock, shaking off excess marinate. Add as many chicken pieces as possible to the pan, while leaving at least 1/2 inch between each.
Grill chicken for approximately 4-5 minutes, or until it has nice golden brown grill marks. Flip and repeat--this side should take a little less time.
Cut into chicken to make sure it's cooked all the way through. If not, add chicken to the prepared baking sheet and place in the oven for 5 minutes.
Allow to rest 10 minutes before serving. Repeat with any remaining chicken cutlets.
Cutlets will keep in the refrigerator tightly sealed up to 4 days, if the chicken was fresh upon purchase.