Bring a saucepan of water to the boil. Add a pinch of salt and the potatoes. Cook for 10–15 minutes until tender.
While the potatoes are cooking, melt the butter in a large frying pan and cook the mushrooms until brown for 2–3 minutes or until brown. Leave aside.
When the potatoes are tender, drain them well, peel and slice them. If the potatoes are very small there is no need to peel them before slicing.
Heat up the mushrooms again and add the potato slices. Cook for 1–2 minutes until hot, then add enough pesto to coat. Serve immediately with some parmesan on top.
Store the remaining pesto in a jar for up to one week in the fridge. It is delicious mixed into mashed potato.