In a large pot or Dutch oven, melt the butter over medium heat.
Add the chopped onion and sauté for 5-7 minutes until soft and translucent.
Stir in the minced garlic and cook for another minute until fragrant.
Bloom the spices by adding the mustard seeds, turmeric, ginger, cayenne, salt, cumin, and cinnamon.
Stir constantly for about 30 seconds until aromatic.
Add the sliced carrots and lemon juice, tossing to coat everything in the spices.
Cook for 2-3 minutes, then pour in 2 cups / 475ml of the water.
Bring to a simmer, then reduce the heat to low, cover, and cook for 25-30 minutes, or until the carrots are fork-tender.
Carefully transfer the carrot mixture to a high-speed blender.
Add the remaining 1½ cups / 355ml (12 fl oz) of water and blend until completely smooth. Return the puréed soup to the pot over low heat.
Gently whisk in the yogurt and honey until fully incorporated.
Heat through gently, but do not let it boil, as this can cause the yogurt to curdle.
Taste the soup and adjust the seasoning to your liking.
Add a few grinds of black pepper, a pinch more salt, or an extra dash of cayenne for more heat if you desire.
Serve immediately, garnished with a generous sprinkle of fresh chopped cilantro.
