Wash the potatoes and parsnips. Peel and trim the parsnips. Cut them in half lengthwise, then into 1 cm slices. Cut the potatoes in quarters. (Tip: keep on the peel as it includes essential iron, fiber, and vitamins, plus it brings a nice chunky texture to the mash).
Peel the garlic, and crush it with the side of your knife. Wash the fresh rosemary, take the leaves off the sprig, and chop it finely.
Combine the potato, parsnip, rosemary, and garlic in a medium saucepan. Fill the pan with 1,5 liters of water and a pinch of salt. Bring to a boil on medium-high heat. (Tip: keep to a simmer by lowering the heat to prevent the mash from being too watery).
Let it simmer for 15-20 minutes until the potato and parsnip are tender at their core (prick with a fork or a small knife to test). Drain the contents in a strainer, put them back into the saucepan, and add the cheese sauce, black pepper, and a drizzle of olive oil (optional). Mash until homogeneous and serve with some freshly chopped herbs of your choice.