In a shallow mixing bowl, whisk together the dried onion flakes, smoked paprika, honey, ketchup, brown sugar, soy sauce, salt, and pepper until smooth and combined.
Add the pounded chicken breasts to the bowl and turn to coat each piece thoroughly in the marinade.
Cover the bowl with plastic wrap or transfer the chicken and marinade to an airtight container or resealable bag. Refrigerate for at least 2 hours and up to 48 hours.
When ready to cook, lightly oil the grill grates or heat a grill pan and preheat to medium-high.
Remove the chicken from the marinade and discard any leftover marinade.
Grill the chicken over medium-high heat about 6–8 minutes per side, or until the internal temperature reaches 165°F (74°C) and juices run clear.
Transfer the cooked chicken to a plate and let it rest a few minutes before serving.