Preheat the oven to 400 degrees Fahrenheit. Line a large rimmed baking sheet with parchment paper for easy cleanup, if desired.

Peel the carrots and cut them on the diagonal so each piece is about ½" thick at the widest part.

Place the carrots on the baking sheet. Add 2 tablespoons olive oil and ½ teaspoon of the salt. Toss until the carrots are lightly coated in oil and seasonings. Arrange them in a single layer.

Roast the carrots until caramelized on the edges and easily pierced through by a fork, 25 to 40 minutes, tossing halfway.

In a Dutch oven or soup pot, warm the remaining 1 tablespoon olive oil over medium heat until shimmering. Add the onion and ¼ teaspoon salt. Cook until the onion is softened and turning translucent, 5 to 7 minutes.

Add the garlic, coriander and cumin. Cook until fragrant, about 30 seconds to 1 minute.

Pour in the vegetable broth and water, scraping up any browned bits on the bottom.

Add the roasted carrots to the pot when they are out of the oven. Bring to a boil, then reduce to a gentle simmer. Cook for 15 minutes.

Remove the pot from heat and let it cool for a few minutes. Transfer the hot soup to a blender in batches.

Add the butter, lemon juice, and several twists of black pepper. Blend until smooth.

Adjust seasoning with salt and pepper, and add more butter or lemon juice if desired. Blend again and serve.