Move the oven rack to the lowest position in the oven, this will allow the spiral ham to be in the center of the oven. Preheat the oven to 275°F, and remove all the packaging from the meat. It's best to allow your fully cooked spiral ham to warm up to room temperature by taking it out of the fridge 1 hour before cooking it. Use an oven thermometer to make sure your oven is correct.
Heat the oil in a small skillet over medium high heat and saute the onion until soft while stirring occasionally. Add the garlic cloves and saute for two minutes, stirring constantly. Transfer the onion and garlic to the roasting pan and add the fresh thyme sprigs and the bay leaf.
Deglaze the skillet with ¼ cup of unsalted chicken broth. Scrape the browned bits off the bottom of the skillet with a wooden turner. Pour the liquid from the skillet into the roasting pan, then pour the rest of the chicken broth into the roasting pan.
Place the spiral ham cut side down on top of the onion slices and cover tightly with foil.
Cook a room temperature spiral ham at 275°F for 12 minutes a pound or until a meat thermometer reads 140°F. A 10 pound spiral ham would take roughly 2 hours. Uncover the ham during the last 30 minutes. (If you want to apply a glaze, check recipe notes for recipe ideas and instructions.) If your spiral ham is cold, cook at 300°F for 10 minutes a pound.
Remove the spiral ham from the oven and tent with foil. Let the ham rest for 20 minutes so the juices can redistribute.
Remove the sliced onion and garlic cloves from the roasting pan and place in a serving bowl. If you didn't glaze your ham, you can also make the pan drippings into a gravy.
Serve the meat with the cooked onions and garlic cloves, spiced applesauce, mustard sauce, or homemade gravy.