Pound the chicken breasts to an even thickness (about 1/2 inch) using a meat mallet or rolling pin, then place them in a gallon freezer bag or a shallow baking pan.
In a mixing bowl, whisk together olive oil, lemon juice, Dijon mustard, honey, dried minced onion, oregano, paprika, minced garlic, cayenne pepper, and salt and black pepper until smooth.
Pour the marinade over the chicken in the bag or pan, seal or cover, and refrigerate for a few hours or overnight to marinate.
When ready to cook, preheat your grill or grill pan to medium-high heat and remove the chicken from the marinade, letting excess drip off.
Grill or sear the chicken until an instant-read thermometer inserted into the thickest part registers 165°F (about 6–8 minutes per side depending on thickness); transfer to a plate and let rest 5 minutes before serving.