Step 1

Melt butter in a saucepan over medium-high heat. Add onion, salt, and tomato paste. Cook, stirring often, until the onion softens and the tomato paste turns a deep brick red or brown color, 5 to 7 minutes.

Step 2

Add garlic, cumin, and cayenne; cook, stirring, for 2 more minutes. Stir in the broth and bring to a simmer. Reduce heat to medium-low and add the lentils, celery, and carrots.

Step 3

Stir and bring to a simmer. Cook, stirring occasionally, until the lentils and vegetables are very tender, 30 to 40 minutes.

Step 4

Meanwhile, grind the mint and salt into a paste using a mortar and pestle. Add lemon zest, lemon juice, and yogurt, and stir together until combined. Refrigerate until ready to serve.

Step 5

Taste the soup and adjust seasoning as needed. Serve as-is or use an immersion blender to puree about half of the soup to achieve a creamier texture. Serve hot with spoonfuls of the lemon-mint yogurt.