Boil water in a big pot and add salt and pasta to it. Cook the pasta until it is tender before you remove it from water. Drain the water to give pasta a rinse before you set aside the shells in order to stuff them. While the pasta is cooking, you can work on the sauce in the meantime.
Now add olive oil to a large skillet and heat it up to medium heat and dice onion in it. While the onions are cooking, slice the sausage and break it into the skillet. By breaking up the sausage thoroughly, cook the sausage mixture. Add fresh arugula and minced garlic when the mixture is cooked. Keep stirring until the arugula is wilted. Only then remove it from the heat.
Put the can of tomatoes in a saucepan and place it over low heat and let it simmer. You can use a spoon to break the tomatoes and add a little amount of sugar, chilli flakes and salt according to your taste. After it has thickened, put the heat to low and turn the oven to 350 F.
Wait for the sausage mixture to cool down then combine with ricotta. Toss it with sausage and arugula and add more salt and pepper if necessary. Put the tomatoes back on medium low heat and add cream and simmer while stirring.
Now take a large baking dish and spread it with 1/3 of the sauce. Stuff each and every shell with the already made sausage mixture and wrap baking sheet around it. Put the sauce on top and sprinkle fresh parsley. Now transfer it to the oven and let it bake for around 20 minutes. Serve hot and you can also add a big salad if you want.