"tsukimi" or "moon viewing" udon was a regular staple when I lived in Kagawa prefecture for 3 years. Kagawa, known for its "Sanuki" udon was cheap, filling, and absolutely delicious. Though this udon is far from the real stuff, it hit the spot on a sunny winter afternoon. Added shiitake mushrooms and the sliced hard boiled egg (and the reason for its name) were just enough to bring about that feeling of 懐かしい or "reminiscence."