Heat the olive oil in a large saucepan or soup pot. Saute the shallots and garlic for 2 or 3 minutes, until fragrant.
Add the leeks, and continue to cook for about 5 minutes until softened.
Add the lemon zest, hot pepper and white wine, and cook until the wine reduces by about a third.
Next, add the lemon juice and chicken stock to the saucepan, and stir to blend the flavors, allowing it to heat for a minute or two.
Add the cauliflower and potatoes, cover, and bring to a bubble.
Add salt and pepper to taste, and allow the mixture to simmer for about 15 to 20 minutes, until the vegetables are softened.
If you like a chunkier soup, you can mash the veggies with a fork. For a smoother mixture, use an immersion blender.
Adjust seasonings, and add the yogurt for a creamier mix.
Heat and serve.