In a medium soup pot or Dutch oven, heat the oil, two turns of the pan, over medium. Add the butter. When it foams, add the leeks and garlic and cook, stirring often, until the vegetables begin to soften, about 2 minutes. Add the broccoli, potato and thyme; season with salt and pepper. Raise the heat a bit, then cover and cook, stirring occasionally, until the vegetables soften, about 5 minutes.
Add the stock; bring to a boil. Add the spinach, season with the nutmeg and stir until the spinach wilts, 2 to 3 minutes. Remove from the heat. Using an immersion blender or working in batches in a food processor, puree the soup, thinning with water if too thick. Add the lemon zest and juice; season. Serve in shallow bowls topped with the creme fraiche, nuts and cheese.