Preheat the oven to 190°C / 375°F / Gas mark 5.
Line 2 baking trays with greaseproof paper and set aside.
Bring a large pot of water to a boil.
Peel squash and slice in half then lengthwise.
Scoop out any seeds in the base of the squash, and slice as thin as you can.
Once sliced, boil (in batches) for 1 minute or so.
Drain and lay flat in a single layer on kitchen paper to dry.
Coat with olive oil and sprinkle generously with salt and pepper.
Sprinkle chopped rosemary on top and bake for 20 minutes.
Check them after 10 minutes to ensure they don’t burn.
Remove from the oven once they start to brown and crisp up, then salt to taste and serve.