1. Preheat your oven to 200°C/Fan 180°C/Gas 6. Fill a large pan with water, pop on a lid and bring to the boil. Trim the top and base off the butternut squash and halve it vertically (no need to peel). Scoop out the seeds and discard them (or you can fry them with spices to make a snack). Slice the squash as thinly as possible. Set aside.
2. Peel the onions and finely slice them. Peel and finely chop the garlic cloves. Pick the thyme leaves off their sprigs.
3. Pour 1 tbsp oil into a medium-sized pan and warm over a medium heat. Slide in the onions, garlic and thyme leaves, keeping a pinch of leaves back for garnishing. Sprinkle in a ittle salt and pepper. Fry for 5 mins, stirring often, till softened.
4. Meanwhile, slide the sliced squash into the pan with the boiling water. Return to the boil and cook for 4-5 mins, till the squash has softened slightly. Drain and leave to cool a little in the colander.
5. Stir the flour into the pan with the onions and cook for a further 2 mins, being careful to stop the flour from catching on the bottom of the pan. Pour in a little bit of the milk and stir till the flour and onions are thick. Slowly pour in the remaining milk, stirring frequently, till combined into a sauce. Bring to a simmer.
6. While the sauce is coming to the simmer, slice the stalks off the chard leaves (or keep them for stir-fries). Finely slice the chard leaves. When the sauce is simmering, stir in the chard leaves to wilt them. Take the pan off the heat and stir in ¾ of the Perl Las.
7. Layer half the squash in an ovenproof dish. Spoon over half the chard and Perl Las sauce. Top with the remaining squash and then top with the sauce. Sprinkle the breadcrumbs over the top. Slide the dish into the oven and bake for 25 mins, till bubbling and golden.
8. While the gratin is cooking, halve the tomatoes and pop them in a bowl. Pour over 1 tbsp balsamic vinegar and 1 tsp oil. Sprinkle with a little salt and pepper and mix. Add the rocket on top of the tomatoes but do not mix in at this point.
9. When the gratin is cooked, divide it between a couple of warm plates. Toss the tomato and rocket salad to dress it, and serve on the side.