In a small bowl, whisk together Dijon mustard, whole-grain mustard, honey, olive oil, apple cider vinegar, minced garlic, salt, and pepper until smooth. Reserve 2 tablespoons for basting.

Place chicken breasts between two sheets of plastic wrap and gently pound to an even 1/2-inch thickness using a rolling pin or heavy skillet. This ensures even cooking.

Place chicken in a shallow dish or zip-top bag. Pour the remaining honey mustard mixture over the chicken, turning to coat. Let marinate at room temperature for 15 minutes (or refrigerate up to 2 hours).

Preheat your grill or grill pan to medium-high heat (about 400°F/200°C). Clean and oil the grates.

Remove chicken from marinade, letting excess drip off. Place chicken on the grill. Cook for 5-6 minutes per side, without moving, until grill marks form and internal temperature reaches 165°F (74°C) on an instant-read thermometer.

During the last 2 minutes of cooking, brush the reserved honey mustard glaze onto both sides of the chicken. Watch carefully to prevent burning.

Transfer chicken to a cutting board and let rest for 5 minutes. Slice against the grain and serve.