Heat the vegetable stock in a small saucepan over medium heat. Once hot, add the saffron to infuse, and keep warm. Heat the olive oil in a large saute pan. Saute the onions until soft. Add the sausages; stirring and sauteing while breaking the meat into small pieces with the back of a wooden spoon.
When the meat starts to brown, add the rosemary and sage, and cook on medium-high heat until fragrant, about 5 minutes. Then add the white wine and stir well until the sharp smell of wine is cooked off, but the pan is not dry. Add the saffron broth.
Reduce the heat to medium-low and simmer for 8 to 10 minutes; make sure you do not over-cook, as the liquid part of the sauce is what will stick to your pasta and give it the distinctive saffron flavor.
Remove the rosemary sprigs. Set aside.
Bring a pot of salted water to a boil and cook the pasta according to the directions on the box. Drain it about 1 minute before what is indicated on the box. Saute in the pan with the sauce over high heat for about a minute or so.
Serve with some freshly grated Parmesan. Drizzle with some extra virgin olive oil, and if you'd like to add a fancy touch, place a piece of the rosemary you set aside on top or on the side of the plate.