Trim the root of the shimeji mushrooms and separate into small clusters. Slice the onion thinly and mince the garlic after removing its core.

Heat olive oil in a frying pan over medium heat. Add garlic, shimeji mushrooms, and onion, and sauté until coated with oil.

Add tuna, canned tomatoes, salt, and pepper to the pan. Mix well, cover, and simmer on low heat for 5 minutes. Remove the lid and simmer for another 5 minutes over medium heat, stirring occasionally.

Boil water in a pot, add salt (not listed in ingredients), and cook spaghetti according to package instructions. Reserve 2 tbsp of pasta water before draining.