Heat 4 tablespoons olive oil in a large pot or Dutch oven over medium-high heat, add the 1 medium yellow onion and cook until softened (about 4 minutes). Stir in the 4 cloves garlic and 1 tablespoon fresh ginger and cook until fragrant, about 1 minute.
Add the 1 cauliflower , 3-4 cups vegetable stock, 1 teaspoon curry powder, and salt and black pepper , then raise the heat to bring to a boil. Cover the pot, lower the heat to medium-low, and simmer until the cauliflower is tender, about 20 minutes.
Use an immersion blender, or carefully transfer the soup to a standard blender, and puree the soup until smooth. Return it to the pot if needed.
Stir in the lime juice if using, then ladle into bowls and serve hot with more lime wedges if desired.
Notes
Add cream. You can absolutely add ¼ cup of heavy cream to the soup at the end for a richer, creamier vegetarian cauliflower soup. Try coconut milk or coconut cream for a vegan version.
Cook it in a dutch oven. If you have a dutch oven, I highly advise using it to cook this soup. In general, dutch ovens are a handy tool to ensure that heat is distributed evenly during cooking. They also hold a pretty large capacity, which is helpful if your cauliflower is quite large (as mine often are).
It's OK to use water. I like using broth for a bit of added flavor, but water works perfectly as well, and is a more economical choice.
Adjust the amount of liquid. Depending on how large your head of cauliflower is, you may need to add more liquid. Conversely, if it's a little small, you may wish to use less liquid so it's not too diluted/watery. Start with 2 cups of liquid and add more if needed. You want to just cover the cauliflower so that it cooks down well. You can always add a bit more later if needed.
Use an immersion blender. If you can, puree the soup right in the pot using an immersion blender so that you don't have to deal with transferring the hot broth and vegetables to a blender or food processor. If you do need to transfer it, do it in batches, very carefully, so you don't get burned.