In a large pot, melt 2 tablespoons of the butter. Add the onion and garlic, season generously with salt and cook over moderately high heat, stirring occasionally, until the onion has softened and is starting to brown, about 8 minutes. Stir in the tomato paste and cook, stirring, until fragrant, about 1 minute. Stir in the cumin and paprika, then add 2 1/2 cups of water and bring to a simmer, scraping up any browned bits on the bottom of the pan. Add carrots, thyme sprigs, thyme leaves, bayleaves, lemon zest and lemon juice. Add 1 tablespoon of salt and 2 1/2 cups of water. Cover and cook on high until the carrots are tender.