Core and cut cauliflower into pieces. Reserve about a cup of small florets.

Melt 1 1/2 tablespoons of butter in a 3-quart saucepan over medium heat. Add leeks, onions, cauliflower, salt, and curry powder. Cover and cook for 20 minutes, stirring occasionally.

Add water, milk, and cream. Increase heat to medium-high and bring to a boil. Reduce heat to a simmer and cook, stirring occasionally and skimming off foam as needed, for 30 minutes. Remove from heat and let cool about 10 minutes.

While the soup is simmering, bring a saucepan of salted water to a boil. Add vinegar and reserved florets and cook until mostly tender, about 5 minutes. Drain.

Working in batches, if necessary, blend soup smooth and strain into clean saucepan. Taste and add salt and pepper as needed. Keep warm over low heat.

Meanwhile, melt remaining 1/2 tablespoon of butter in a large skillet over medium heat. Add florets and saute until browned in a few spots. Add to soup either before or after serving.

Top bowls of soup with croutons.