In a heavy-based pot, heat the olive oil and butter until quite hot. Add the leeks, onions, and salt, and then turn the heat to medium low. Add the thyme and bay leaf. Cook slowly, stirring occasionally, until the vegetables are slightly softened and a bit browned, 10 to 15 minutes. Add the saffron, cayenne, and garlic; cook, stirring, for 1 minute. Add the potatoes and chicken broth. Increase the heat and bring to a boil. Reduce the heat to a bare simmer, cover, and cook until the potatoes are tender, about 15 minutes. Discard the thyme sprig and bay leaf. Transfer 1 cup of the soup to a blender to puree and then return the puree to the pot. Adjust the seasonings. Ladle into shallow bowls; garnish with the parsley, creme fraiche, and pepper.