Meanwhile fry the diced onion in the olive oil until translucent. Then add the butter, melt it and stir in the flour. Deglaze with the milk (if you like it more liquid, you can add a little more milk if necessary). Stir well, after about 1 minute add the tuna and let it simmer over a medium heat while the spaghetti cooks al dente. Shortly before the end of the spaghetti cooking, add the peas to the sauce (the sauce shouldn`t boil afterwards so that the peas don`t disintegrate!) And season everything with salt, pepper and nutmeg.
Arrange the spaghetti with the sauce on the plates and serve.