Pat the shrimp dry and add cajun seasoning. In a pot at medium-high heat, add ½ tbsp olive oil. Once hot, sear shrimp until opaque (this means they're cooked), about 2 minutes per side. Set aside. Do not overcook or you'll get dry, dense, chewy shrimp.
In the same pot without rinsing/cleaning (whatever is stuck to the pot is flavor), add the remaining ½ tbsp olive oil. Once hot, add in your onions and cook until transparent, about 3 minutes. Stir occasionally to make sure it doesn't burn or stick to the pot. Then add the garlic until aromatic, about 2 minutes.
Add in the heads of the shrimp and stir until orange. Carefully scrape the bottom of the pan so the bits stuck to the bottom gets lifted and adds to the flavor.
Pour in 700ml of the kombu dashi stock or water and boil. Reduce to simmer without a lid for about 5 minutes to let the flavors blend together.
Meanwhile, assemble your bowls with the noodles at the bottom and the toppings arranged on top.
In your simmering pot, press the shrimp heads to let out its juices using a fork or a spoon. Don't skip this step as it's flavor!
Add in 1tsp MSG and salt to taste.
Strain the soup to remove all the solids (onion, garlic, shrimp parts, etc.) and add to the bowls. Serve and eat hot!