Prepare all ingredients by chopping the onion, dicing the carrots and apple, and mincing the garlic. Measure out the spices and liquids so everything is ready to go.
Place the chicken chunks into the crockpot, spreading them evenly across the bottom.
Add the chopped onion, diced carrots, and minced garlic on top of the chicken, distributing evenly.
Sprinkle the curry powder over all the ingredients, letting the fragrant aroma start to fill the kitchen.
Pour the rinsed lentils, diced apple, chicken broth, and coconut milk into the crockpot, stirring gently to combine everything thoroughly.
Cover the crockpot and set it to low. Let it cook undisturbed for 6 to 8 hours, until the chicken is tender and the lentils are soft and starting to break down.
After cooking, open the lid and taste the soup. Adjust the seasoning with salt and pepper as needed. If the soup is too thick, stir in a splash more broth to loosen it.
Use a ladle to serve the hot, creamy Mulligatawny into bowls. Garnish with fresh herbs or a squeeze of lemon if desired for extra brightness.