Preheat the oven to 375 degrees F. Line a baking sheet with foil and put a baking rack on top.
Mix together the grill seasoning, dressing, and chopped herbs in a small bowl. Reserve 2 tablespoons. Generously rub the hens inside and out with the herb and dressing mixture. Stuff each hen with a sprig of each herb and a piece of lemon.
In a large bowl, combine the carrots, onions and potatoes with the reserved 2 tablespoons of dressing mixture, a generous pinch of salt and pepper. Put the vegetables in a large roasting pan and arrange the hens on top. Roast them until the juices run clear and an instant-read thermometer registers 180 degrees F when inserted into the thickest part of the thigh, about 50 to 55 minutes. Remove the pan from the oven, cover with foil and let the hens rest for 5 minutes before serving. Transfer the hens to a large serving tray with the roasted vegetables.