Cook the ribbon noodles in plenty of salted water until they are al dente. Cut the tuna into thumb-sized cubes and marinate in soy sauce. Fry quickly in a little olive oil for 1 - 2 minutes.
Put the spinach leaves in a pan with a little butter and heat. Season to taste with nutmeg, salt and pepper. Add the mascarpone and creme fraiche and dilute with a little pasta cooking water.
Remove from the heat, mix in the parmesan, season with salt, pepper and possibly a little lemon juice. Mix in the tuna cubes and serve with the ribbon noodles.