Heat 1 tablespoon neutral oil in a large pot over medium heat. Once hot, add the shrimp shells and tails and sauté for 5 minutes until pink. Mash the shells lightly into the side of the pot as they cook.
Add the sliced yellow onion, crushed garlic, crushed ginger, and Thai chili peppers. Sauté for 3 to 5 minutes until fragrant.
Pour in 8 cups of water and add 2 teaspoons kosher salt. Bring to a boil. Reduce the heat to low and simmer, uncovered, for 30 minutes.
After the stock has simmered, turn off the heat and strain into a large bowl through a fine mesh sieve. Discard solids and set the stock aside. Wipe out the pot and return it to the stove.
Prepare the soup:
Bring a large pot of water to a boil for the noodles.
While the water comes to a boil, heat 2 teaspoons sesame oil in the pot used to make the stock over medium heat. Once hot, add the sliced onion and shiitake mushrooms. Season with a pinch of salt and sauté for 5 minutes until the mushrooms begin to brown around the edges. Add more oil if the mushrooms stick.
Once the water is boiling, add the noodles and cook according to package directions. Drain and transfer to a bowl. Toss with the remaining 1 teaspoon sesame oil to prevent sticking.
Add 1 tablespoon shichimi togarashi to the mushrooms and onions and sauté for 1 minute. Pour in the prepared stock and bring to a boil. Reduce the heat to low.
Finish the soup:
Add the broccoli florets and the reserved, peeled shrimp. Add a pinch of salt. Cover the pot and simmer for 5 to 6 minutes or until the shrimp are opaque and the broccoli is bright green. Cooking time will vary depending on the size of your shrimp and broccoli florets. Remove the pot from the heat.
To serve:
Divide the cooked noodles between bowls and ladle the soup on top. Garnish with minced scallions, shichimi togarashi, or chili oil as desired. Enjoy!
Notes
Note 1: If using fresh noodles, you’ll need 16 to 20 ounces total. I recommend two (10-ounce) packages of Sun Ramen Kaedama. For dry noodles, you’ll need anywhere from 8 to 12 ounces total, as they expand much more as they boil. Try it with dry somen, udon, soba, or even thin rice noodles. You can also use frozen udon instead.Note 2: If using stem-on shiitake mushrooms, you can scrub the stems and add them to the stock with the shrimp shells to give it more umami flavor.