Remove Cornish hens from the package, rinse, and pat dry with paper towels. Split the birds in half before seasoning if desired. I cut them after marinating. There is no wrong or right way of doing this. They both work.
Season Cornish hen with salt and pepper and place them in a large bowl or Ziplock bag.
Whisk the olive oil, Dijon mustard, soy sauce, minced thyme and garlic, chopped parsley, lemon juice, and pepper flakes in a medium mixing bowl.
Pour the marinade over the Cornish hens and thoroughly mix to coat every inch with the marinade.
Let the hens marinate in the refrigerator for at least 30 minutes or up to 24 hours. The longer, the better (within reason) for the marinade to infuse the hens.
When ready to grill, remove the Cornish hens from the marinade, shake off excess particles, and discard the remaining marinade.
Preheat your grill to medium-high heat 375-400°F (190-204°C).
Place the Cornish hens on the preheated grill and cook for 5 minutes per side, long enough for it to char without burning.
Reduce the temperature to medium heat 350℉ (180℃) and continue cooking until the meat is no longer pink and the juices run clear (about 30 minutes). Use a meat thermometer for best results. The internal temperature should reach 165°F (74°C) at the thickest part.
Remove the hens from the grill and let them rest for 5-10 minutes before serving.
Enjoy your delicious grilled Cornish hens with the chosen sides.