1. Preheat oven to 475F.
2. Put pork bones in a large roasting pan and sprinkle lightly with salt and pepper. Put the pan in the oven and roast until pork is golden brown, about 45 minutes. Turn the bones once or twice so they brown evenly.
3. Transfer the pork to a large pot and add water to cover by about 2 inches. Drain and discard fat from roasting pan, then put the pan over medium heat. add 1 cup water and scrape up the brown bits from the bottom of the pan. Add this deglazing liquid to the stock pot. Bring the water to a boil over high heat, then reduce the heat to maintain a simmer. Skim and discard the fat and foam that rises to the surface. add the onion, celery, carrot, bay, and thyme to the pot and continue to simmer for 2 hours, adding water as necessary to keep the pork covered.
4. Strain the stock through a fine sieve in a large storage container; discard solids. Set the stock aside to cool completely. Cover and refrigerate for 5 days or freeze for up to 3 months.
