Cook the spaghetti according to the package instructions, ensuring it is cooked al dente.
Prepare the meitako. If using whole mentaiko in sacs, use a spoon to scrape out the caviar/roe, discarding the sacs.
Heat a skillet over low heat. Add the butter and olive oil, followed by the mentaiko, dashi, heavy whipping cream, mirin, salt, and ground black pepper. Bring to a light simmer, stirring to combine the sauce well.
Drain the spaghetti and transfer it to the skillet. Mix well to combine with the sauce.
Add the egg yolk, if using, and stir to make the pasta richer and creamier.
Dish out the pasta and garnish with the meitako, some microgreens, and a sprinkle of ground black pepper. Serve immediately.