In a small saucepan, bring the stock and saffron to a simmer over medium-high. Reduce the heat to low; let the saffron steep in the stock while you heat up the pasta water and start the sauce.
Bring a large pot of water to a boil for the pasta.
In a large skillet, heat 1 tbsp. olive oil, one turn of the pan, over medium-high. Add the sausage and cook, stirring often and breaking up with a spoon, until browned, about 6 minutes. Transfer the sausage to a paper towel-lined plate. Add the remaining 1 tbsp. oil, one turn of the pan, to the skillet. Add the asparagus and cook, stirring often, until lightly browned, 2 to 3 minutes. Add the shallots, thyme, garlic and lemon zest; season with salt and pepper. Cook, stirring often, until the shallots soften, 1 or 2 minutes more. Stir in the wine, then the saffron stock. In a small bowl, using a fork or your fingers, knead the butter and flour together until completely combined; whisk into the sauce. Return the sausage to the skillet, reduce the heat to low and let simmer, stirring occasionally, until the sauce thickens, about 2 minutes.
Salt the boiling water, add the pasta and cook until almost al dente. Reserve 1/2 cup of the starchy cooking water; drain the pasta. Add the pasta and the cooking water to the sauce; toss to combine. Season, then drizzle with EVOO. Pass the cheese at the table for topping.