Heat the oil in a large, heavy bottomed pot over a medium low heat. Add the onions and sauté for 5-7 minutes, or until the onions are clear and soft. Add a few tablespoons of water as you cook them if necessary to prevent sticking. Add the garlic and cook for 1-2 more minutes, or until the garlic is fragrant, but not yet burning.
Add the red curry paste, turmeric, and salt to the soup. Mix the spices up with the onions and garlic until everything is well incorporated; continue to cook over medium low heat for 30 more seconds.
Add the cauliflower florets and vegetable broth to the pot and turn the heat to high. Bring the mixture to a boil and the turn the heat to low. Cover and simmer for 25 minutes, or until the cauliflower is entirely tender.
Use an immersion blender to blend the soup until it's totally smooth, or transfer it in batches to your blender to do the job. If you do use the standing blender, be careful, as hot soups tend to spatter aggressively.
Return the blended soup to the pot, if you used a standing blender. If you feel that the soup is too thick (more like a purée than a soup), stir in 1-2 cups of water to achieve a consistency that you like.
Stir in the cashew cream or coconut milk, along with the lime juice. Taste the soup, then adjust the lime juice and salt to taste.
Garnish and serve. The soup will keep for up to five days in the fridge, and it can be frozen for up to two months.