The last I did have ramen at an American restaurant was eons ago at the now defunct Modus Supper Club. Made for a nice light chicken noodle soup but as a ramen an utter failure. The one bellowing at the UnderBelly however I could tell would be a lot more memorable in a good way. With all that pork, how can it not be!?? ;)
But first, a couple things to get out of the way... There is this annoyance of a "no spoons available" policy here. There was some funny reads about it on Chowhound but I think the creators of UnderBelly might be mistaken with maybe how Miso Soup is typically had.(?) Miso soup is never served with a spoon in Japan. Chopsticks in your good hand, you pick up the piping hot lacquer bowl with the other and sip directly. Ramen on the other hand while also occasionally carefully sipped directly from the bowl lip is almost always served with a spoon. If there already isn't one perched in your bowl you'd be sure to find them available clustered on the counter or table. My point: There is no reasonable reason for UnderBelly to not offer spoons.
The other was that they need to train the staff to say the word properly. It's Ton-k"o"tsu, not Ton-k"a"tsu, the latter - deep fried panko crusted pork cutlet. My friends make the mistake all the time, but since it is easy to see a considerable amount of effort being put into their flagship meal I feel it's only right.

Having said that I would almost bet UnderBelly's ramen is technically a Tonkotsu-Shoyu. The tinted color is a hint (though pure tonkotsu soups aren't necessarily always milky white) but I thought there was a nuance of sweet soy sauce present. May have leached from the toppings but it is a nice way to cut some of the funk associated with a 100% pure pork bone tonkotsu. Santouka uses a little marine based umami to accomplish this (characteristic of Hokkaido, Asahikawa ramen) while yet others can include the addition of chicken bones to mellow.
The soup is pretty creamy rich, my first more so than the second, and at least in the beginning not too salty. The amount of oil on the surface seemed more conservative than in the photo of Kirk's bowl but either cases not uncommon for this genre of extra hearty ramen, sometimes further including speckles of seabura back fat. Can safely be categorized as Kotteri without a doubt while not quite Cho-kotteri / Cho-noukou - the status reserved for even yet muddier doro-doro viscous and fat laden creations. This second visit the temperature was much better (hotter) than the first but I still felt they can crank up the Fahrenheit considerably. If you happen to wear glasses, at least real ones with lenses in them, the steam should easily cloud your vision after the first sip while you then blow air to the chopsticked noodles for dear mercy moments before slurping.
These noodles are a thicker side medium, wavy chijire style. They were also executed better this time which the first seemed like the noodles may have been dunked in water that hadn't quite reached a rolling boil. Rather limp and starchy, but the noodles in my second bowl today was near al dente proper. Otherwise fairly standard as for flavor goes if not still on the starchy side and lacking true resiliency (in Japan helped with the use of kansui).

To go with the very hip open and airy design of the restaurant the selection of toppings at UB should also be considered out of the box. As curious as I am I may have to pass on the Grilled Unagi but Braised Oxtail and Bacon Wrapped Mushroom does sound interesting. In the "UnderBelly Ramen" shown, along with melty Char-Siu and soft boiled egg, ten-bucks also gets you a few thick slices of Applewood Smoked Bacon and a Berkshire pork sausage link. While the juicy link admittedly puts things in porky land overdrive, it personally made me reminisce the many quick sustenance type bowls I created during college years and I probably would prefer them more as a side (which are available the way with mustard dip and kimchi). The egg if marinated was very light in flavor. Of these soft boiled I personally prefer ones where the yolks are set a little firmer (not runny) but they were a welcome as they always are, especially near the end as the soup edged toward salty with the steeping bacon and sausage. The grated ginger also helped cut some of the rich monotony as I finished supported along with blanched crunchy bean sprouts and Wakame seaweed.

My first bowl was the $12 "Belly of the Beast" that came with Oxtail Dumplings, Hoisin glazed Short Rib and Smoked Brisket. While the starchy noodles and soup temperature wasn't helping out then I have to say I enjoyed more the less expensive house bowl had today. The topping flavors didn't seem to clash as much as to my expectations of ramen complements (in tonkotsu) and while I know this is Asian-Fusion, the gummy skinned Shrimp Gyozas weren't as good eats as they should've been despite their beautiful magazine page worthy presentation.
Even without the few glitches I personally felt the two bowls were a bit heavy handed for my taste and something difficult to see myself having repeats of. Still I'm pretty sure I will be back to try their Vegetable Ramen (with white asparagus and truffle oil) and also the Charred Kimchi Ramen (mmm). That, Santouka doesn't have and relates to why UnderBelly gets huge kudos for trying something new in downtown. With a few tweaks I feel they have the underpinnings of becoming something much greater. Again, with that much involvement of delicious pork how can it not be? And maybe one day they'll even offer you a spoon.
UnderBelly, 750 W. Fir St, San Diego, CA 92101