Spray a saucepan with low calorie cooking spray and set on a medium heat. Add the onions and cook for 25-30 minutes, stirring frequently to prevent colouring.
Add the chicken stock pot, white wine stock pot, water and thyme and bring to a simmer, then reduce heat and continue cooking for 15 minutes.
Remove the thyme sprigs, then blitz the soup with a stick blender until smooth. Add the crème fraiche, then season to taste with salt and pepper before serving.