Heat your oven to 200°C/Fan 180°C/Gas 6. Scrub the potatoes. Pierce them all over with a skewer or fork.
Pop them on a plate and bung in your microwave on high for 10-15 mins. When they’re tender to the touch, use an oven glove or tongs to put them on a baking tray.
Drizzle each potato with ½ tbsp oil and sprinkle with salt and pepper. Bake in the oven for 30 mins till cooked through (or bake for 1 hr 30 mins if you don’t have a microwave). Test them with a skewer to check they’re soft all the way through.
While the potatoes are cooking, put the spinach. in a dry pan. Cook over a medium heat for 1 2 mins till the spinach has just wilted. Drain in a colander. Press the spinach with a wooden spoon to squeeze out as much liquid as possible. Finely chop it.
Separate the egg yolk from its white. Whisk the milk, egg yolk, mustard and some black pepper in a small pan. Grate the cheese and add to the pan. Add the spinach. Heat gently and stir for about 2 mins till the cheese has melted into a smooth sauce.
When the potatoes are cooked, halve and scoop out their flesh. Pop into a bowl and mash. Heat your grill to medium. Line a grill pan with foil.
Add half the sauce to the mash. Mix well. Spoon the cheesey mash back into the potato skins. Top with the remaining sauce. Grill for 5 mins till golden brown and bubbling.
Whisk 1 tbsp of the vinegar with 1 tbsp oil and a pinch of salt and pepper. Toss the salad leaves with the dressing. Serve the rarebit jackets with the salad.