Prep. Preheat your oven to 450ºF. Place the rosemary, thyme, onion, garlic, and lemon slices on a baking sheet in a single layer and set aside.
Prepare the Cornish hen. Pat the Cornish hens dry with paper towels. Lift the skin of the Cornish hens with a finger and carefully place 2 slices of butter under the skin of each hen. I place one slice right over each breast.
Make the spice rub. Combine the salt, pepper, Italian seasoning, paprika, garlic powder, and red pepper flakes in a bowl. Season the Cornish hens with the spice rub.
Get ready to roast. Place the Cornish hens over the lemon slices and aromatics on the baking sheet, and drizzle everything with olive oil. Twist the wings to tuck under the bird, and you could also tie the legs with twine, but that's optional.
Roast. Reduce the oven temperature to 400˚F and bake the Cornish hens for 55-60 minutes, or until the internal temperature of each hen reaches 165ºF.
Let them rest, then serve. Remove the hens from the oven and let them rest for 10 to 15 minutes before serving. Serve with some extra lemon wedges on the side.
Notes
Make sure your baking pan is wide enough: If the Cornish hens are touching, the skin will not get crispy.
Check that they're done: The Cornish hens are fully baked when the inner temperature of the thickest part reaches 165F.
Get more color: If you prefer darker skin on the hens, broil them for an extra couple of minutes at the end.
Add herbs: For extra flavor, you can fill the cavity of the hens with herbs and garlic.