Heat the oil in a frying pan then cook and stir the chopped onion until translucent, about 10 minutes. Set the onion aside to cool.
Preheat an oven to 180 degrees C. Grease an oblong glass baking dish.
Fill a bowl with warm water then soak the challah, one slice at a time. Gently squeeze out the excess water then place the challah in a large bowl. Mix in the cooked onion, carrots and celery then gently mix in the eggs. Season with salt and pepper.
Spread stuffing in the prepared baking dish, cover with aluminium foil then bake in the preheated oven until cooked through and set, about 1 hour.
Use this stuffing to dress a turkey or chicken, stuff and bake according to your favourite recipe and the weight of the bird.