Gather all your equipment and ingredients: chop the onion, carrots, and celery into small, even pieces; mince the garlic; and cut the chicken thighs into bite-sized chunks.
In a skillet over medium heat, toast the cumin seeds until fragrant, about 1 minute, filling your kitchen with a warm aroma. Add the oil, then sauté the chopped onion, carrots, and celery for about 5 minutes until they soften and become fragrant, with a slight golden hue forming.
Add the minced garlic and ground turmeric to the skillet, cooking for another minute until the garlic is fragrant and the spices are evenly coated on the vegetables.
Transfer the sautéed mixture to your slow cooker. Add the chicken chunks, rinsed lentils, coconut milk, chicken broth, garam masala, and a pinch of salt. Stir everything gently to combine.
Set the slow cooker to low and cook for 6 to 8 hours. During this time, the chicken becomes tender, the lentils break down into a silky texture, and the fragrant spices meld into a warm, inviting broth.
Once cooking is complete, check the consistency. If it's too thick, stir in a splash of warm broth; if it’s too thin, uncover and cook on high for 15 minutes to thicken slightly. Taste and adjust salt as needed, and add a squeeze of lemon juice for brightness if desired.
Ladle the hot mulligatawny into bowls, garnish with chopped cilantro if using, and serve with naan or crusty bread for a cozy, comforting meal.
