Remove the Cornish hens from the package, rinse, and pat dry with paper towels (see notes). You can spatchcock the birds or split them in half, as I did.
Season them with salt and pepper and place them in a large bowl or Ziploc bag.
Whisk the olive oil, Dijon mustard, soy sauce, garlic, parsley, thyme, paprika, lemon juice, and pepper flakes in a medium mixing bowl.
Pour the marinade over the chicken, coat well, and marinate them in the refrigerator for around 30 minutes.
When ready to grill, take the Cornish hens out of the marinade, shake off the excess, and discard the remaining marinade. If you want to reuse the marinade for something, make sure to cook it long enough to kill the bacteria from raw meat.
Preheat the grill on medium-high heat to 375-400°F (190-205°C).
Cook the Cornish hens on the grill for 5 minutes per side, long enough for them to get good grill marks without burning them.
Lower the temperature to medium, 350℉ (180℃), and continue grilling until the chicken is no longer pink inside and the juices run clear (30-45 minutes). Use an instant-read thermometer to ensure it reaches 165°F (74°C) in the thickest part of the thigh and breast.
Remove them from the grill and let them rest for 5-10 minutes before serving.
Enjoy with your favorite sides.