Clean thoroughly the calamari and remove the tentacles, leaving the tubes clean and intact. Set the tubes aside.
Boil the peeled potatoes in salted water for about 20 minutes, or until a fork can pierce them easily. Drain the potatoes, mash them and set them aside to cool down.
Drizzle a pan with olive oil and sautè the garlic clove until it starts to brown. Add the chopped squid tentacles to the pan and sautè a couple of minutes. Remove the garlic and add the chopped bread.
Let it brown for a couple of minutes, then pour a dash of white wine and it evaporate completely.
Remove from heat and let it cool down completely. Add the mashed potatoes, the egg, the breadcrumbs, the milk and the finely chopped parsley.
Mix well and season with salt, black pepper, chili pepper flakes and a pinch of ground nutmeg.
With a spoon or with your hands, stuff the calamari, leaving at least 2-3 cm (1 inch) from the top, as the squids will shrink a lot while cooking. Close the top with a toothpick.
Make the Sauce
In a large pan, drizzle some olive oil and sautè the garlic cloves for a couple of minutes. Add the squids and brown on all sides, then set them aside.
Add the tomatoes and season with salt, black pepper and chili pepper flakes. (If too acidic, add also a teaspoon of sugar).
Turn down the heat and let the sauce simmer for 15-20 minutes, then add the calamari back in the pan and cook for another 15-20 minutes.
Let it rest for about 5 minutes before serving (otherwise the filling will be too soft), the add a generous amount of finely chopped parsley and enjoy!