Peel and finely chop the onion. Peel and dice the carrot. Trim and finely slice the celery. Warm a large pan for 1 min. Add 1 tbsp oil and the onion, carrot and celery. Season with a little salt and pepper. Cover. Turn the heat down and sweat for 10 mins, stirring now and then, till the onion is soft and glossy.
While the veg sweat, halve the squash. Scoop out the seeds. Peel and finely chop the flesh. Peel and crush or grate the garlic. Add the squash and garlic to the pan with 2 tsp garam masala. Crumble in the stock cube and pour in 850ml boiling water (or use 850ml hot homemade stock).
Add the lentils. Pop the lid back on. Turn up the heat. Bring to the boil. Turn the heat back down to medium and simmer for 15 mins till the squash is very tender.
Taste and add more salt and pepper if you think it needs it. Ladle into warm bowls and serve.