Boil the spaghetti in boiling water.
Remove the skin from the mentaiko and prepare only the roe.
Finely chop the onion and garlic.
Add olive oil to a pan and sauté the garlic and onion.
Pour in the heavy cream and bring to a simmer.
Add the cooked pasta and mentaiko, and mix well while reducing the sauce.
Season with salt and pepper.
Serve on a plate and garnish with parsley or seaweed flakes.