Remove any excess fat from the cavity of each guinea fowl. Wash thoroughly inside and out and pat dry with paper towels. Rub the cavity with a little salt. Cut off the two ends of the oranges, stand them on end and carefully slice off the skin (once you have removed one piece of skin you can see where the flesh meets the skin). Slice the oranges into five or six rounds each. Remove the tougher outside ribs of the celery until you reach the white, dense bulb and slice across thinly.
Put in a bowl, mix in the thyme and a small pinch of salt and pepper, then stuff the cavity of each guinea fowl with this filling. Pull the skin at the front of each guinea fowl's cavity forward, to cover the filling, and tightly tie/truss up.
Heat a thick-bottomed pan and add the olive oil and the guinea fowl, the skin of which has been rubbed in sea salt and pepper. Cook until lightly golden on all sides, then add the garlic, butter and sage and cook for 3-4 minutes until golden brown. Add the wine at intervals, enough to keep the pan slightly moist at all times. Place in the oven for 45 minutes, checking every 10-15 minutes and just topping up the wine as necessary. The guinea fowl will be roasted and partially steamed.
When cooked, carefully remove from the oven and place upside down on a dish, allowing all the juices and moisture to relax back into the breast meat for at least 5 minutes. While your meat is resting, make the gravy.
Remove all the fat from the roasting pan and place the pan on gentle heat. In the bottom of the pan will be your cooked, soft, sweet, whole garlic cloves and some gorgeous sticky stuff-when this gets hot, scoop out the stuffing from the guinea fowl cavity and add to the pan with about 2/3 cup of wine. As the wine boils and steams, scrape all the goodness with a spoon from the bottom of the pan into the liquor. When it has all dissolved, leave to simmer gently. Squash the cooked garlic out of their skins with a spoon (discard the skins); this will also thicken the gravy slightly, as well as give it flavor. Pour any of the juices that have drained out of the rested birds into the pan with the gravy, simmer and season to taste. Serve the guinea fowl with roast potatoes and any simply cooked green vegetable-spinach, kale, bok choy or broccoli.