Place the carrots, parsnips, and sweet potatoes in a large roasting tin that is non-stick. Spray them with low-calorie cooking spray. Mix them well, then evenly sprinkle the cumin seeds and garam masala over them. Lightly season with salt and pepper. Roast for 50 minutes, turning everything and adding the peppers halfway through.
While the vegetables are roasting, cook the rogan josh sauce according to the instructions on the pot.
When the vegetables have 20 minutes left to cook, take a large non-stick pan and heat it over medium heat. Add the cherry tomatoes to the pan and spray them with low-calorie cooking spray. Gently cook them for 2-3 minutes, stirring occasionally, until the tomatoes burst. Add the rogan josh sauce, lentils, stock, and yogurt to the pan. Stir well to combine. Bring the mixture to a boil, then reduce the heat to a simmer and cook for 15 minutes, stirring occasionally.
Once the vegetables in the oven are ready, transfer them to the pan with the sauce. Stir in the chopped coriander. Transfer the dish to a serving dish and garnish with the reserved coriander sprigs. Serve with plain boiled rice and a salad made from diced cucumber, tomato, chopped coriander, and a squeeze of lime juice.
