Place the rosemary and garlic in the cavity of the guinea hen.
Rub the bird inside and out with the herbed salt, then truss it.
Heat the olive oil in a deep casserole large enough to hold the bird.
Sear the bird in the oil on top of the stove to brown in lightly on all sides.
Remove the bird from the pan and scatter the slab bacon, carrot, celery, and onion in the casserole.
Preheat the oven to 425°F (220°C). Place the bird on the vegetables in the casserole.
Sprinkle the wine and stock around it and scatter 3 teaspoons of the peppercorns casserole.
Cover the casserole and place in the oven.
Roast for 45 minutes.
Remove the guinea hen from the casserole and strain the pan juices into a saucepan.
Heat the sauce and add the remaining whole peppercorns.
Season to taste with salt. You should have about 1½ cups (375 ml) of sauce.
Remove the trussing from the guinea hen and quarter it.
Arrange it on a platter with the vegetables and serve with the sauce alongside.