Preheat the oven to 200°C. Line a large baking sheet with parchment paper for easy cleanup, if desired.
Peel and chop the carrots into small, even pieces. The smaller the pieces, the faster they'll roast.
Place the carrots on the baking sheet. Add 2 tbsp of extra virgin olive oil and ½ tsp fine sea salt. Mix until the carrots are lightly coated in oil and seasoning. Arrange them in a single layer.
Roast the carrots until caramelized around the edges and easily pierced with a fork, 35 to 40 minutes, turning halfway through cooking.
When the carrots are almost roasted, in a large pot, heat 1 tbsp of extra-virgin olive oil over medium heat. Add 1 medium chopped onion and sauté until it becomes translucent and fragrant, about 5 minutes. Add the 2 minced garlic cloves to the pot and stir to combine with the onions. Sprinkle in ¼ tsp fine sea salt, ½ tsp of ground coriander and ¼ teaspoon ground cumin powder. Sauté until fragrant, stirring frequently, about 30 seconds to 1 minute.
Add 4 cups of vegetable broth and 2 cups of water to the pot, while scraping up any browned bits from the bottom with a wooden spoon or sturdy silicone spatula.
Add the roasted carrots to the pan when they are out of the oven. Bring the mixture to a boil over high heat, then reduce the heat to a simmer. Cover and cook for 15 minutes, to give the flavors time to blend.
Using an immersion blender or a regular blender, carefully puree the soup until it is smooth and velvety. If using a regular blender, allow the soup to cool slightly before blending, and blend in batches if needed. (Do not fill beyond the maximum fill line or the soup may overflow!)
Return the pureed soup to the pot over low heat. Stir in 1 to 2 tbsp unsalted butter and 1 ½ tsp lemon juice. Taste and adjust the seasoning with more salt or black pepper if necessary.
Ladle the Roasted Carrot and Coriander Soup into bowls, garnish with a sprinkle of black pepper, and serve hot. You can also add a drizzle of olive oil or a dollop of sour cream for extra richness.