Place the oil and butter into a medium-sized pot and set over medium heat. When the butter is melted, add the fennel, onion and garlic and cook until softened, about 5 minutes. Mix in flour and tomato paste and cook 2 minutes more. While stirring, slowly pour in the stock. Mix in the diced tomatoes. Bring the soup to a simmer. Simmer 10 minutes, or until the fennel is very tender. Puree the soup in a food processor or blender, or in the pot with a handheld immersion blender. Return the soup to a simmer; season with salt and pepper. To serve, ladle the soup into bowls, top with nuggets of the goat cheese and garnish with sprigs of fresh fennel leaves.