In a large Dutch oven or heavy-bottomed pot, melt the remaining 2 tablespoons butter over medium heat. Add the remaining onion, thyme, garlic, and bay leaf and the saffron threads and cook, stirring, until the onions are translucent. Add the cauliflower and cook, stirring, for another 5 minutes. Add the reserved cooking liquid and the remaining chicken stock. Simmer, stirring occasionally, for 30 minutes, or until the cauliflower is very tender. Add the heavy cream and crème fraîche and bring to a simmer. Discard the bay leaf and thyme and transfer the mixture to a blender (you will need to do this in batches). Puree until smooth and season with salt and pepper. Return to the pan, bring to a simmer, and stir in the mussels just before serving.