Allow the toasted bread to sit out for approximately 24 hours, until hard.
Preheat oven to 160 C / Gas mark 3. Lightly grease a 23x33cm (9x13 in) baking dish.
Crush the bread into crumbs with a rolling pin. Place the crumbs in a large bowl.
Melt the butter in a medium saucepan over medium heat. Stir in the onion and celery and slowly cook until soft. Remove from heat and drain.
Mix the eggs and vegetable stock into the breadcrumbs. The mixture should be moist, but not mushy. Use water, if necessary, to attain desired consistency. Mix in the onion, celery, sage, salt and pepper.
Press the mixture into the baking dish. Bake 1 hour in the preheated oven, or until the top is brown and crisp.