Peel and finely chop the onion and garlic. Sauté in butter until soft, sprinkle the flour over the vegetables, and stir to combine. Deglze with the white wine, add the saffron threads, and stir in the broth. Simmer for 10 minutes, stirring occasionally. Add in the cream, and puree the sauce with an immersion blender.
If too thin, reduce until you reach the desired consistency. Add a bit of broth if the sauce is too thick. Add in the creme fraiche at the end, and season to taste with salt and pepper.