Heat the butter in a Dutch oven or saucepan over medium heat. Add the onion and cook, stirring occasionally, until softened, 5–7 minutes. Stir in the curry powder and a pinch of salt and pepper, and cook for 1 minute until fragrant.

Add the broth, water, and carrots to the pot. Bring to a boil, then reduce the heat to low. Cover and simmer until the carrots are tender and easily pierced with a fork, about 20 minutes.

Using an immersion blender or transferring in batches to a blender, carefully puree the soup until smooth.

Return the soup to the pot, adding water as needed to adjust to your desired consistency. Season generously with salt and pepper, stir in the lemon juice, and serve warm.