Wash the greens, shake off the excess water, then cut off the ends and compost them or save for soup – as a general guide, I cut away any stalks that are thicker than the base of my little finger.
Shred the leaves into 3cm- to 4cm-wide strips.
Put half the butter and half the oil in each of two large, deep frying pans and put them on a medium heat.
When the butter has melted and started to foam, distribute the leaves and salt between the two pans and cook, stirring occasionally, for 25-30 minutes, until the greens have given up all hope of freshness and turned forest-black, glossy and soft.
Tip all the leaves into one of the pans, toss through the grated cheese, then take off the heat.
Slice open the focaccia horizontally, then evenly pile the greens on the bottom half.
Slap the lid on top and compress.
If you like a bit of theatre, tightly wrap the focaccia in foil, pop it in a bread tin and pack with a large bread knife; once on location, turn out and slice with panache.
Or, more sensibly, slice into portions before you leave and wrap individually.
