Heat the oil in a large skillet over medium-high heat. Add the onion, celery, carrots, and garlic, and sauté until the vegetables are tender, around eight minutes. Add the mushrooms and sauté until soft and most of the liquid is absorbed, three to five minutes. Remove from heat and allow to cool for five to 10 minutes. Add the lightly beaten egg, and stir to combine.

Remove the matzoh bag from the box. Gently break the matzoh in the bag until you have one-inch pieces. Very gently stir the matzoh into the sautéed vegetables. Add the chicken stock and pepper, then gently stir to combine. Immediately transfer the stuffing to a whole chicken and roast.