1. Heat the olive oil and butter in a large pot over medium heat, and saute the leeks, cauliflower, garlic, and thyme for about 10 minutes. Stir in the vegetable broth, and bring the mixture to a boil. Reduce heat, cover, and simmer 45 minutes.
2. Remove the soup from heat. Blend the soup with an immersion blender or hand mixer. If the soup is too thin, add oatmeal to thicken. Season with salt and pepper, and lemon juice, if using.