Preheat oven to 400℉.
Peel carrots, then cut them on the diagonal so each piece is about ½" thick.
Place carrots in large bowl. Add 2 tablespoons olive oil and ½ salt. Toss until carrots are lightly coated. Arrange them in a single layer on a baking sheet.
Roast carrots until they are caramelized on the edges and easily pieced with with a fork, 25-40 minutes. Toss about halfway through.
When carrots are almost done roasting, warm the remaining 1 tablespoon olive oil in a soup pot over medium heat until shimmering. Add onion and ¼ teaspoon salt. Cook, stirring occasionally, until onion has softened and is turning translucent, 5-7 minutes.
Add garlic, coriander, cumin, and garlic. Cook until fragrant, stirring constantly, 30 seconds to 1 minute.
Pour in broth (and additional water if needed). Scrape up any browned bits on the bottom.
Add roasted carrots to the pot. Bring the mixture to a boil over high heat. Reduce heat to maintain gentle simmer. Cook, partially covered, 15 minutes.
Once soup is done cooking, remove pot from heat and let cool.
Once soup is cool, transfer to a blender.
Add lemon juice and several twists of black pepper.
Blend until completely smooth. Add more salt, pepper, and lemon to taste.
