IN A LARGE HEAVY-BOTTOMED POT, combine the oil and coriander seeds and toast the seeds over medium heat, stirring frequently, until fragrant, about 1 minute.
Add the onion and garlic, season with 1 teaspoon of the salt and a few cracks of pepper sir, and cook until the onions are translucent, 4 to 5 minutes.
Add the carrots, the remaining teaspoon of salt, and a few cracks of pepper and stir to coat. Stir in the butter and toss the carrots until the butter has completely melted. Add the brown sugar and stir until it dissolves. Pour in the stock, bring the mixture to a simmer, and cook until the carrots are just tender, 8 to 10 minutes. Remove the pot from the heat.
Working in batches if necessary, carefully transfer the hot soup to a blender and blend until smooth. Strain the soup through a fine-mesh strainer back into the pot. Taste for seasoning, adding more salt and/or pepper if needed. Serve hot.