Clay pots were the original ovens; earthenware vessels used by the Romans thousands of years ago. Some modern clay pots are glazed for easier clean up; others are made of unglazed terra cotta, similar to the ancient earthenware used by the Romans. Clay pots retain moisture and juices, similar to slow cookers, but they also speed up the cooking time, similar to convection ovens. Meat and vegetables can be cooked together in a clay pot, creating a one pot meal with easy clean up.
Soak the top and bottom of the clay pot according to the manufacturer's instructions, prior to using it. These directions will vary; some glazed bottoms may not require soaking. If you don't have the instructions, soak both the top and bottom in cold water for 15 minutes.
Combine onions, potatoes and parsley and place on the bottom of the clay pot.
Mix the salt, pepper, and thyme, and rub on the pork. Rub the cut sides of the garlic cloves on the pork, and then insert the cloves in slits in the meat made with a paring knife.
Place the roast on top of the vegetables, and pour the chicken stock around the edges.
Place the clay pot in the oven. Cook the roast at 218 degrees Celsius for 2 1/2 hours, or until a meat thermometer reads 73.9 degrees C. Allow the roast to sit for 20 minutes before carving.
Vary the seasonings and vegetables as desired. Trim excess fat from the meat before placing in the clay pot.
Do not preheat the oven, as putting the clay pot into a hot oven could crack the pot.