An immersion blender, also known as a stick blender, is the easiest way to puree tomatoes right in their can. Slideshow: More Meaty Pasta Recipes
Heat the olive oil in a saucepan over moderate heat. Add the onion and cook, stirring occasionally, until softened, 6 to 8 minutes. Add the prosciutto and cook, stirring occasionally, until browned, 3 to 5 minutes. Add the tomatoes, season with salt and pepper, and bring to a simmer. Simmer until slightly thickened and well-flavored, about 30 minutes. Stir in the cream and keep warm over low heat.
Meanwhile, cook the spaghetti in a large pot of boiling well-salted water until al dente. Drain well. Top the spaghetti with the prosciutto sauce and serve immediately.