Step 1 Cut the potatoes into bite-size pieces and place in a pot of simmering water fitted with a steamer or rack. Steam until tender, about 15 minutes.
Step 2 Remove the sausage meat from the skin and crumble it into a mixing bowl. Add the bread crumbs and 1 tablespoon parsley and beat together with a wooden spoon until well mixed. It will form a pretty tight ball. Add the egg and continue mixing until the egg is well incorporated. The mixture will loosen up a lot.
Step 3Fill the squid tubes with the sausage mixture. You can do this with your fingers or with a small spoon, but by far the easiest way is to use a pastry bag. Fill the squid no more than half full; the mixture will expand during cooking. Depending on the size of the squid, there may be some sausage mixture left over. Seal the openings with toothpicks.
Step 4Heat the olive oil over medium-high heat. When the oil is quite hot, add the stuffed squid and cook on both sides until the surface begins to color, about 2 minutes total.
Step 5Remove the squid to a plate and keep warm. Empty all but about 1 tablespoon of the oil from the skillet, leaving behind any browned bits that have stuck to the bottom of the pan.
Step 6Add the onion and cook until soft, about 3 minutes. Add the garlic and cook until it's fragrant, about 2 minutes. Add crushed red pepper flakes and the wine and stir, scraping the bottom of the pan to free any of those browned bits. Cook until the wine reduces to a syrup, about 5 minutes.
Step 7Add the tomatoes and juice and cook until they begin to thicken, about 3 minutes. Add the capers and season with salt and pepper.
Step 8Return the squid to the pan, reduce the heat to low, cover with a lid and simmer gently 15 minutes. Add the cooked potatoes, replace the lid, and simmer until the squid can be easily pierced with a knife, about 15 minutes more.
Step 9Spoon into warmed broad soup or pasta bowls and sprinkle with more parsley. Remove toothpicks and serve immediately.