Place the potatoes on a microwave-proof plate and pierce several times with a sharp knife, then cook for 5 minutes in a microwave. Turn and cook again for a further 3 minutes, then transfer to the top of a preheated oven, 220°C (425°F), Gas Mark 7, for 5 minutes to golden a little.
Meanwhile, place the spinach in a saucepan with the measurement water and heat over a medium heat for 5 minutes until wilted.
Remove the potatoes from the oven, cut in half lengthways and scoop out most of the flesh, reserving the skins. Put the potato into a bowl and season with pepper. Mash and stir with the pesto, then fold in the spinach leaves.
Place the potato halves on a baking sheet, pile the pesto potato back into the skins and scatter with the Parmesan. Cook under a preheated grill for 5 minutes until golden and bubbling. Serve with baked beans.