Preheat the oven to 200°C (gas mark 6, 180°C fan). Pierce the potatoes all over with a fork, then roll in the oil and a little salt on a baking tray. Bake for 1hr until crisp and golden. Set aside until cool enough to handle. Preheat the grill to medium-high.
While your potatoes are going golden, mix the egg yolk, mustard and cheese in a bowl and set aside.
Once cooled, halve the cooked potatoes and use a teaspoon to scoop out the flesh into a bowl – leaving 0.5cm around the edges to support the skins. Mash the flesh with a fork, season, then fold through the spring onions, chives and half the mustard mix.
Spoon this back into the skins. Spread the remaining mix over the top of the potatoes and grill for 2 – until golden and just starting to catch.