In a small bowl, whisk together the oyster sauce, chili garlic sauce, salt, pepper and water. Set aside.
In another larger bowl add the flank steak, garlic, cornstarch, egg yolk, salt, pepper then toss everything together.
In a large skillet or wok heat 1 tbsp of the peanut oil. Add about half of the beef and stir-fry it for about 3 minutes or until it starts to brown. You will need to do this in a couple batches as you don't want the steak pieces to stick to each other. Also if you need more oil after the first batch feel free to add some more. Transfer the beef to a plate and set aside.
In the same skillet, heat the remaining tbsp of peanut oil. Add the onion and shiitake mushrooms to the skillet and cook for about 5 minutes until the onion softens and the mushrooms start to brown.
Add the pepper, broccoli and 1/3 cup of water to the skillet and cook for another 8 minutes or so until the broccoli is tender-crisp.
Add the noodles, beef and oyster sauce to the skillet and toss well. Cook for another minute until the sauce thickens a bit, then serve.
* I usually buy my udon noodles cooked, all that's required is to soak them in hot water for a couple minutes until they loosen up. They usually come in 7 oz pkgs or 20 oz larger packages. In this recipe I used 3 packages of 7 oz. If you can't find them, any thick asian style noodle will work.Oyster sauce is a very dark sauce made from sugar, salt and thickened with cornstarch, then flavored with oyster essence. You should be able to find it in the international aisle at your grocery store. Hoisin would make a good substitute.Any cut of steak will work for this recipe as long as it's cut thin. A good trick to getting thin slices is to place the steak in the freezer for 10-15 minutes before cutting.These noodles will keep for 3 - 4 days in the fridge if stored in an airtight container. I wouldn't recommend freezing this dish as the noodles don't thaw well after time in the freezer.Please keep in mind that nutritional information is a rough estimate and can vary greatly based on products used.