Add the potatoes to a large pan and cover with cold water. Add salt like you would with pasta and bring to a boil, then reduce to a simmer. After 15 mins, the potatoes should be cooked through.

Meanwhile, peel and finely slice the red onion into rings. Add to a small bowl along with the zest and juice of a lemon and a pinch of salt.

Halve the cherry tomatoes and set aside.

Once the potatoes are cooked, drain and transfer to a large bowl. Cube in the butter, add the harissa and mix well until the butter melts.

Add the onions and lemon juice along with the cherry tomatoes, a pinch of salt and some black pepper. Toss again and then transfer to a platter.

Finely chop the chives and sprinkle over the top.