Bring a large pot of water to a boil with 1 tablespoon of salt. Cook the pasta according to the package instructions.
Remove a few pieces of sea urchin and set them aside for topping later.
Meanwhile, prepare the sauce. Finely chop the onion and garlic. Heat oil in a frying pan over medium heat. Add the onion and garlic, then cook for a couple of minutes. You may need to remove the pan from heat briefly to adjust timing with the pasta.
Two minutes before the pasta is done, add the sea urchin and sake to the onion mixture, and cook for a minute. Stir in the heavy cream, and lower the heat to avoid boiling.
Season with salt and pepper. Add the drained pasta to the sauce and coat it thoroughly. Adjust the taste with extra salt and pepper if needed.