Whilst the squash is baking in the oven you can make the cavolo nero pesto. Cut the leaves away either side of the stem, and discard the stalk. Rinse the leaves under cold running water. Place the leaves in a steamer, I use a bamboo basket, lined with parchment paper placed over a saucepan of boiling water. Bring the water to the boil and steam the cavolo nero for 10 minutes. Remove from the heat, leave to cool. Place the cooled wilted leaves in the palm of your hand and squeeze out the excess moisture in to a bowl. Alternatively, you can place in a clean tea towel and squeeze through, (discard the water).