Bring a large pot of water to a boil, alt it, add the pasta and cook until al dente. Drain, reserving a little of the pasta cooking water.
while the pasta is working, in a large, deep skillet combine the stock (or water), wine, onion, carrot, celery, garlic, citrus zest and saffron; season lightly with salt if using stock, or more heavily if using water. Bring to a boil and let reduce very rapidly to 1 1/2 cups, 15 to 20 minutes. Strain and return to the pan. Stir in the butter to melt over low heat.
Add the pasta, poppy seeds and some cheese to the sauce. Toss for 1 to 2 minutes to combine. Serve with the remaining cheese and garnish with a few extra threads of saffron.