Halve the squash and scoop out the seeds. Pick through them and save a small handful, discarding the rest and picking off all the fleshy pulp. Add them to a small pan with a lug of olive oil and fry until golden. Keep to one side.
In a pestle and mortar, bash the fennel and coriander seeds and dried chilli flakes with 1 teaspoon of sea salt and a pinch of black pepper until fine.
Cut the squash halves into wedges and add to a roasting tray. Tip over the spices, then scatter over the cinnamon. Crumble the chestnuts directly into the tray, then drizzle lightly with olive oil. Toss to coat.
Roast in the oven to cook for 30 to 45 minutes, or until beautiful, golden, and soft when you pinch it.
When the squash is done, sprinkle over a little salt from a height and carefully tip the golden seeds into the tray, leaving the oil behind in the pan.
Pick the rosemary leaves onto a board with a good pinch of salt and pepper and finely chop it all together. Roll the beef about on the board until it’s well coated in the rosemary.
Return the pan of oil to a high heat, and once its screaming hot, add the beef. Sear it for 1 to 1½ minutes, so it browns, then turn it over and do the same on the other side. Remember you’re just searing it to get a bit of colour on the outside, not cooking it – it should be blue in the middle.
Meanwhile, make the dressing. Add the mustard, 6 tablespoons of extra virgin olive oil and the lemon juice to a bowl or mortar and mix well. Have a little taste and check the seasoning then put aside.
With a really sharp knife, slice the beef up as thinly as you can without hacking it up or ruining it! Go thicker to do it nicely if you need to. Use the back of a knife to carefully flatten out each slice of beef, then transfer them to a large platter.
Sprinkle with a little salt, as the seasoning won’t have got through to the centre of the beef. Arrange the squash pieces over the top, then crumble over the roasted chestnuts and scatter over the seeds.
Drizzle everything with a bit dressing (save any leftovers for another day), then lightly dress the salad leaves and pop these on the top. Take it to the table with a block of Parmesan for shaving over.
If you wanted, you could cook your squash ahead of time and simply reheat it when you’re ready to eat. The beef can be seared ahead of time, too.