Preheat the oven to 400°F (200°C).
Pat the Cornish hens dry with paper towels; truss legs with kitchen twine if desired.
In a small bowl, combine unsalted butter, salt, and black pepper; chop rosemary and thyme leaves and mix into the butter.
Loosen the skin over the breast and thigh of each hen, spread half the herb butter under the skin, then rub remaining butter all over the outside.
Stuff each hen’s cavity with half a lemon, two smashed garlic cloves, one sprig of rosemary, and one sprig of thyme.
Optionally drizzle olive oil in a roasting pan; place hens on a rack or directly in the pan.
Roast at 400°F for 20 minutes, then reduce oven temperature to 375°F and roast for an additional 30–40 minutes, or until thigh temperature reaches 165°F.
Remove hens from the oven and let rest 5–10 minutes before serving.