Start by preparing your ingredients: dice the onion, carrots, celery, and apple; chop the chicken into bite-sized pieces; and measure out your spices and liquids.
In a skillet, heat the oil over medium heat until it shimmers. Add the diced onion, carrots, and celery, and sauté for about 5-7 minutes until the vegetables are fragrant and slightly softened, with the onions becoming translucent and beginning to caramelize.
While the vegetables cook, toast the curry powder and turmeric in the same skillet for about 1 minute until fragrant—this helps release their essential oils and deepens their flavor.
Add the toasted spices to the sautéed vegetables, stirring for 30 seconds to coat evenly and enhance the aroma.
Transfer the seasoned vegetables to your slow cooker. Add the chicken pieces, rinsed lentils, coconut milk, chicken broth, and diced apple if using. Stir everything together until well combined.
Set your slow cooker to low and cook for 6 to 8 hours, or on high for 3 to 4 hours. The chicken should become tender and easily shred, while the lentils break down and thicken the broth, filling your kitchen with warm, spicy aroma.
About 15 minutes before serving, stir in the fresh lemon juice to brighten the flavors. Taste the soup and add salt and pepper as needed to balance the spice and acidity.
Once the soup is ready, ladle it into bowls, garnish with fresh herbs if desired, and serve hot. The result is a thick, fragrant, and comforting Mulligatawny with a velvety texture and a gentle spice kick.
