Cheesy Mushroom and Pancetta Bread Pudding

Name: Cheesy Mushroom and Pancetta Bread Pudding
Ingredients: 3 tbsp. rendered bacon fat or unsalted butter, divided; 2 medium yellow onions, thinly sliced (2 ¼ cups); Salt and freshly ground black pepper; 3⁄4 lb. mixed, cleaned wild mushrooms such as chanterelles, shiitake, hen-of-the-woods, and cremini (about 6 cups); One 2 ½ inch thick slice pancetta, diced (⅔ cup); 1 tbsp. fresh thyme leaves; 2 tbsp. dry white wine; 2 large eggs plus 1 large egg yolk; 3⁄4 cup whole milk; Pinch freshly grated nutmeg; Pinch ground cayenne; 8 ½ inch thick slices soft white bread, cut in ½-inch cubes (about 7 cups); 1 cup shredded Gruyère or Comté (4 oz.); 1⁄4 cup crème fraiche