Put a medium skillet over a medium flame and heat the oil. Add the onion and cook until translucent, about 5–7 minutes. Add the meat to the pan and season with salt, cooking until browned, about 10–12 minutes. Add the wine and let it cook for about 1 minute. Stir in the tomato paste, water, salt, and basil.
Lower the flame to a simmer and cook, stirring occasionally, for about 15 minutes.
Preheat your oven to 400 degrees.
Cook the tagliatelle in generously salted boiling water for about half the time of the package instructions. Drain and transfer directly to the sauce, mixing well to coat the pasta evenly.
Line a tube pan with the prosciutto slices, making sure they overlap and hang over the edges of the pan. Add half of the pasta mixture to the pan, then add the mozzarella. Top with the remaining pasta.
Fold the prosciutto slices over the top to seal everything in and bake for 40 minutes.
Let the timballo rest in the pan for 20 minutes before carefully flipping it onto a platter. Slice and serve.