I got the idea of caramelizing sugar and then adding mustard oil to begin a Bengali curry from Monisha Bharadwaj. From that point on the curry is my own.
In a large pan, sprinkle the sugar with a few drops of water over medium high heat. When it starts to bubble, stir it and let it caramelize until a rich medium brown.
Add the mustard oil with the whole cloves, cinnamon sticks, and mustard seeds. Partially cover the pan to prevent the popping mustard seeds from escaping.
When the mustard seeds have slowed in their popping, remove the lid and add the bay leaves and cumin seeds. Let roast until browned and fragrant, which will happen quickly.
Add the chopped onion and stir it into the spices. Sprinkle with a pinch of salt. Let cook, stirring occasionally, until the onions have begun to caramelize. Keep a little water by the cooktop to sprinkle over the onions if they start to stick or scorch.
Add the ginger, garlic, tomato paste, minced chile pepper and turmeric. Stir and let the onions continue to caramelize another 5 minutes.
Add the ground cumin, ground coriander and 1 teaspoon garam masala. Stir and let the spices roast briefly.
Add the ground beef. Break up the clumps and stir it into the onions and spices. Let cook for 2-3 minutes and then reduce the heat to medium. Continue cooking until the ground beef is browned.
When the beef is browned, mix in the potatoes with a pinch of salt. Let them get a little browned by cooking for 5 minutes without stirring.
After the 5 minutes, add the squash, tomatoes and corn with a pinch of salt. Cook, stirring, until the tomatoes start to break down and release their juices, about 5 minutes.
Add the 1 1/2 cups of water and bring to a boil. If your curry looks super saucy, let it simmer uncovered. If it looks like there is no liquid to spare (it certainly should not look dry), cover the pan. Either way let simmer 15-20 minutes, until the potatoes are fork tender and the zucchini is crisp-tender.
Turn the heat to low.
Remove several spoonfuls of the curry and mix it into the yogurt. When the yogurt is quite warm, stir it into the curry with 2 teaspoons of garam masala and the lemon or lime juice. Stir to incorporate.
Taste for sugar, more salt, more garam masala or more lemon or lime juice. Add the cilantro if using (I had just a little to garnish my pictures with from a struggling plant outside so I am sure the dish will be ok without it). Serve hot with rice or flatbread (we almost always opt for Basmati rice).