start with stale, dry bread. You can dry out the bread by either cutting it into cubes and leaving on the counter overnight, or toasting it in the oven at 250°F for 45 minutes.
melt butter in a large pan. Add the celery, onion, apples, sage and rosemary, and cook for 10-15 minutes, until softened. Remove from heat and allow to cool.
Beat together the eggs, then mix in the chicken stock and salt in the bottom of a large bowl. Toss the bread cubes and apple/celery mixture in the egg mixture. Scoop the apple stuffing into the casserole/baking dish.
cover with foil or a reusable silicone cover, and bake at 350°F for 45 minutes. Remove from oven and allow to cool uncovered.