Spinach-rolled chicken with onion purée & charred leeks

• 200 g/7 oz spinach leaves
• 4 skinless chicken breasts

Rinse and drain the spinach, then put it in a saucepan with just the water clinging to the leaves. Put over a medium heat until softened, then drain.

Put one chicken breast in between two sheets of baking paper and gently beat to flatten. Lay a double layer of cling film (plastic wrap) on the work surface, put the flat chicken breast on top, season with salt and pepper and top with some