Add all brine ingredients to a saucepan and boil for 10 minutes to allow sugar and salt to fully dissolve.
Place brine in the fridge to cool. - These first steps can be done ahead.
Cut out the backbone of each Cornish hen with a strong kitchen shears so they are able to lie flat.
Place thawed Cornish hens in a gallon storage bag and pour in brine and ensure birds are fully submerged.
Let the Cornish hens brine for 2 hours.
Remove from bag, rinse under cold water and pat dry.
Grilling
Place hens on grate over hot charcoal, breast side up and let cook till the bottom begins to blacken. The time will vary depending on the heat of your coals.
Flip so the breast side is down and let cook till the skin crisps and begin to blacken.
Use a meat thermometer to check for doneness. If the meat has not reached 165 degrees move the birds away from direct heat so they can continue cooking without burning.
Let rest 10 minutes and serve along with a dollop of wasabi.
Oven Roasting
Preheat your oven temperature to 400 degrees.
Set the spatchcocked hen on a wire rack placed in a rimming baking sheet or roasting pan and place it on the center rack of your oven. Bake for approximately 40 minutes, or until the meat temperature reaches 165 degrees.
Remove from the oven and let rest for 10 minutes, then serve.
Notes
Baking time in the oven will vary depending on the size of your hen, which is why I recommend using a digital oven thermometer to monitor cooking progress.