In the bowl: Per the menu: "house marinated ikura over salmon sashimi & thin noodles." There's also shiso and all-important dashi.
Supporting cast/What to do: That's the serving, so enjoy it for what it is, along with the pretty bowl and the gorgeous garden in the background.
Noodling around: Actually, it would be a bit unusual to order this somen on its own. It should be part of a fuller kaiseki meal. There's talk of a formal kaiseki menu coming to Momiji, but it's yet to materialize, so you'll need to cobble one together on your own.
That said, it's nice to see somen on a menu, as Japanese restaurants are more likely to showcase ramen, udon, and soba. Somen are very thin wheat noodles that, as here, are typically served cold. I eat them often as a cool summer treat with tsuyu, which is a dipping sauce. In this preparation, though, they're in dashi, a broth most commonly made from katsuobushi (fermented bonito) and kombu (kelp). To me, the dashi is the key element of this dish, as it showcases the skills of the chef, here delivered with quality. The somen itself contributes least to the flavor of the dish, as it's more of a vehicle to bring the dashi to your mouth--and to provide slippery texture.
It's fun to have both salmon and "future salmon" with the noodles--the ikura (salmon roe) providing bubbly bursts of sweetness and saltiness. And don't forget to eat that shiso leaf; it's far from a garnish.
If you want more: As mentioned above, the somen is part of the kaiseki menu, most likely one of the first dishes of a proper kaiseki meal. This is a dollhouse-sized treat, so you'll definitely want and need to eat more. By all means, avoid the crazy sushi rolls and order other dishes from the kaiseki menu. I like the goma tofu ($6)--white sesame tofu with dengaku miso paste and candied peanut--which has comforting texture and earthy flavor.
Be aware/beware: Go early if you want to enjoy the solitude of the garden before the noisy Capitol Hill crowds come. They're the ones who'll be eating the sushi rolls. Your kaiseki dish (or dishes) deserves your dedicated attention.