Heat oven to 200C/180C fan/gas 6. Toss the sweet potato and cauliflower with the garam masala, half the oil and some seasoning. Spread out on a large roasting tray. Add the garlic and roast for 30-35 mins until cooked.
Meanwhile, put the lentils in a saucepan with 400ml cold water. Bring to the boil, then simmer for 20-25 mins until the lentils are cooked but still have some bite. Drain.
Remove the garlic cloves from the tray and squish them with the blade of your knife. Put the garlic in a large bowl with the remaining oil, ginger, mustard, a pinch of sugar and one-third of the lime juice. Whisk, then tip in the warm lentils, stir and season to taste. Coarsely grate the carrots, shred the cabbage and roughly chop the coriander. Squeeze over the remaining lime juice and season to taste.
Divide the lentil mixture between four bowls (or four containers if saving and chilling). Top each serving with a quarter of the carrot slaw and a quarter of the sweet potato and cauliflower mix.