Slice the onion thinly, mince the garlic, and chop the green onion.

Boil water in a pot, add salt, and cook spaghetti 1 minute less than package instructions. Reserve 50cc of pasta water and drain the rest.

Heat salad oil in a frying pan over medium heat, add garlic and sauté until fragrant. Add onion and cook until softened.

Add tuna to the pan and mix well. Pour in reserved pasta water, soy sauce, and dashi powder, then bring to a boil.