1. Preheat oven to 350 degrees F. Coat a 9" x 13" baking pan with cooking spray.
2. In a 12" non-stick saute pan, saute the shiitake mushrooms in 1 tablespoon olive oil over medium-high heat until browned and moisture has evaporated (6-8 minutes).
3. Add 1/4 teaspoon salt, reduce heat to medium, and stirring frequently, continue cooking until mushrooms are dark brown and caramelized (6-8 minutes). Move the mushrooms to a large bowl and set aside.
4. To the same pan, add remaining 1 tablespoon olive oil. Saute leeks and celery with 1/2 teaspoon salt until soft (6-7 minutes). Transfer to bowl with mushrooms.
5. In the same pan, add stock, thyme, rosemary, and sage and simmer for 1 minute.
6. Transfer stock to the bowl of vegetables. Stir in the parsley, chives, and pepper. Set aside.
7. Beat the egg and egg white together with milk and remaining 1/4 teaspoon salt. Stir egg/milk mixture and bread into the vegetables. Transfer to prepared baking dish and bake until set and brown (30-35 minutes).