Peel and finely chop the onion. Warm a large pan for 1 min. Add 1 tbsp oil and the onion. 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 onion sweats, 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 1 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 scatter with the pumpkin seeds to serve.