Think of this recipe as a dinner-party building block: People-pleasing and delicious, it will become your signature dish.
Place parsley, rosemary, thyme, garlic, lemon, salt, and pepper in a food processor. Blend until finely chopped. With machine running, add olive oil.
Loosen skin from the hens. Put 1 tablespoon of herb mixture under skin of each hen, smoothing to cover breasts. Place the birds on a large rimmed baking sheet, cover with plastic wrap, and refrigerate overnight.
Remove hens from refrigerator and bring to room temperature (about 1 hour). Preheat oven to 400 degrees.
Place hens on a clean rimmed baking sheet (it's okay if they touch). Brush butter over surface of hens and sprinkle generously with salt and pepper. Roast in oven, rotating pan once until golden brown and cooked through (meat thermometer should read 165 degrees when placed in the thigh), about 50 minutes.
Remove from oven and let stand at least 15 minutes before placing on individual plates. No carving necessary!