Place the dried shiitake in a small bowl with hot water. Let soak while you prepare the other ingredients.
Slice the fresh mushrooms into strips about 1 cm thick.
In a large bowl, whisk together the garlic, olive oil, soy sauce, and mirin.
Add the fresh mushrooms to the bowl and toss to coat. Leave to marinate for 10 minutes.
Cook the spaghetti in salted boiling water until al dente. Reserve 1 cup of pasta water before draining.
Heat a large frying pan over medium heat. Add the marinated mushrooms and cook for 5 minutes until softened.
Remove the rehydrated shiitake from their soaking liquid. Squeeze out excess liquid, slice thinly, and return both the slices and broth to the pan. Simmer for 2–3 minutes.
Add the spaghetti to the pan and toss with the mushrooms and broth. Add reserved pasta water if needed to loosen.
Serve topped with ou nori or shredded nori, and a sprinkle of shichimi togarashi. Add natto if desired.
Notes
Mushroom swaps: Works with any fresh mushrooms. Shiitake, chestnut, oyster, or enoki are great choices, but baby button or closed cup mushrooms from the supermarket also work well.
Dried shiitake tip: The soaking liquid becomes a quick mushroom broth. Let them soak longer for more depth and squeeze well before slicing. Use pre-prepared sushi mushrooms to save time.
Pasta swaps: Can be made with penne, rigatoni, fusilli, or linguine instead of spaghetti.
Storage: Store leftovers in an airtight container in the fridge for up to 3 days. Reheat gently in a pan with a splash of water or soy sauce. Reheat only once.