Preheat your oven to 200°C/Fan 180°C/Gas 6. Peel and chop the butternut squash into 1-2cm small cubes, discarding the seeds. Tumble into a roasting tin and toss with 1 tbsp olive oil. Season. Roast for 30 mins till tender and a little charred. When it's ready, turn the oven off to keep the squash warm while the risotto cooks.
While the squash is roasting, trim the roots and top 3cm off the leek. Halve it and rinse out any grit, then finely slice it. Peel and grate the garlic. Finely chop the rosemary leaves.
Crumble the stock cube into a heatproof jug and stir in 850ml boiling water to dissolve.
Warm ½ the butter in a large pan. Add the leek. Season with a pinch of pepper. Fry over a gentle heat for about 5 mins. Keep stirring so it doesn't pick up too much colour. Add the garlic and most of the rosemary (keep a little back for garnishing), fry for another 1-2 mins.
Stir the risotto rice into the leeks. Add a splash of hot stock, about 100ml. Cook over a medium heat till the rice has absorbed the stock. Stir every few mins while the risotto is cooking.
Keeping adding the stock a splash at a time. Stir and let the rice absorb it before you add the next ladle. Carry on till you've used almost all the stock and have around 100ml left.
While the risotto finishes cooking set a small frying pan on the heat. Toast the pine nuts for 2-3 mins till golden. Tip into a bowl.
Zest and juice the lemon. Remove the squash from the oven. Mash half of the squash with a fork till you have a chunky purée.
Stir in the squash, lemon zest and half the lemon juice. Stir through the remaining butter to melt. Taste and add more lemon juice, salt or pepper if you think it needs it. Add a little remaining stock to loosen the risotto if necessary. Spoon the risotto into warm bowls and top with the toasted pine nuts and remaining rosemary leaves to serve.