Place the tomatoes in a large pot or bowl. Pour boiling water over them, ensuring they are fully submerged. Leave for 10-15 minutes until the skins start to break.
Remove the tomatoes from the water, peel off the skins, and chop them, discarding the hard white core.
In a large, heavy-bottomed pot, heat 30 ml of extra virgin olive oil over medium heat. Add the chopped tomatoes, salt, and dried oregano.
Bring to a boil, then reduce the heat to low and simmer for 1-1.5 hours, stirring occasionally, until most of the liquid has evaporated and the sauce thickens.
Cook the Sausages and Pasta:
While the tomato reduction is simmering, heat 1-2 tbsp of olive oil in a large skillet over medium heat. Add the diced onions and sauté until soft and translucent, about 5 minutes.
Add the minced garlic and cook for another minute until fragrant.
Add the sliced sausages to the skillet and cook until browned and fully cooked, about 8-10 minutes. Set aside.
Cook the spaghetti in a pot of salted boiling water according to package instructions until al dente. Reserve about 1/2 cup of pasta water, then drain the pasta.
Combine and Serve:
Add the sausage mixture to the tomato reduction, and stir to combine. If the sauce is too thick, use a little reserved pasta water to reach the desired consistency.
Add the cooked spaghetti to the sauce and toss to coat well.
Serve hot, garnished with freshly grated Parmesan cheese, black pepper, and fresh basil leaves if desired.