Mulligatawny is an old British adaptation of the traditional combinations of dal (lentils), spices and vegetables in India. Like all such recipes, there are endless versions. This is a basic recipe that you can vary as you please.
The apple and celery would not have been traditional in India, and while yams are often used, they tend to be in recipes that focus on the yams themselves, rather than being just one of a medley of vegetables. But the addition of apple, celery and sweet potatoes has become common for mulligatawny, and I think they do add to the mixture of taste.
Don't be put off by the long list of ingredients. Like many Indian recipes, this one contains a large variety of spices. But most of them are easily available in the average grocery store, and they're all available in any Indian/Asian grocery store and online (including from us, of course). We do not carry the black mustard seeds right now, but plan to add them in the future. In the meantime, they're easy to find in most supermarkets.
One very handy spice container is an Indian masala dabba, which you'll see in one of the photographs. There is a common 7-spice assortment that most Indian cooks will keep in their dabba, but you can use it for whatever spices you most often use. The jars in mine have a little spoon in each one that holds approximately 1/4 tsp.
4 tbsp butter or ghee divided in half or 1/4 cup oil (also divided into two portions)
Chopped spices and vegetables
3 tablespoon grated fresh ginger (or 1 tsp ginger powder)
4 cloves garlic, minced (you can use more or less garlic, as taste dictates)
1 large onion, finely chopped
1 large or two medium carrots, finely chopped
1 celery stalk, finely chopped
1 medium to large apple, chopped
1 medium sweet potato, chopped
1 or 2 small Roma-type tomatoes, chopped
One pound, more or less, chopped chicken (thighs or whatever you have)
1/2 medium jalapeƱo pepper or one whole birds-eye (Thai) green chilli pepper, minced
Ground spices and remaining ingredients for soup
1 tablespoon ground coriander
2 teaspoon ground cumin
2 teaspoon ground turmeric
8 cups chicken or vegetable stock (two of the 1-quart shelf-stable cartons)
1 cup lentils (red lentils-masoor dal-are traditional, but you can use any kind)
Finishing
Salt and pepper to taste
1 cup canned coconut milk (optional)
3 teaspoon fresh lemon or lime juice (I prefer lime juice, but either one is optional)
Yogurt and chopped fresh cilantro, to garnish
In the interest of time, I used my small food processor for most of the vegetables rather than chopping them. That gave a rather different texture to the soup than you will get with larger pieces of the vegetables. I haven't decided whether to do that again, but it did save time.
Heat 2 tablespoons oil, butter or ghee (or any combination of those) in a frying pan and add the whole spices (cumin, coriander, mustard). Heat until fragrant.
Heat the remaining butter or oil in a heavy bottom saucepan. Cook the chicken until just beginning to turn brown. Add Jalapeno pepper, tomato, ginger, garlic, onion, carrot and celery. Cook until the garlic and onion are translucent.
Add the powdered spices (cumin, coriander and turmeric) and cook for two or three minutes more.
At this point, the mixtures could be placed in a slow cooker, or the slow cooker setting in an electric pressure cooker such as an Instant Pot. Do not use the pressure setting!
Add the chicken or vegetable stock, the apple, sweet potato and lentils and increase heat just until the liquid begins to simmer (occasional bubbles, not a hard roiling boil). Cook until lentils and sweet potato are tender, about 45-50 minutes. Add coconut milk and lemon juice if desired (the coconut milk will make the soup more creamy). Season with salt and pepper to taste.
Serve with plain yogurt and finely chopped fresh cilantro, and if you like, with bread. A crusty bread such as you would serve with spaghetti would be appropriate, or the more traditional naan or paratha.