Method
1. Fill and boil your kettle, then fill a large pan with the water, topping it up if necessary. Add a pinch of salt and bring the pan back to the boil. Meanwhile, scrub the Jersey Royals (no need to peel) and slice them into bite-sized chunks. When the water is boiling, add the potatoes and simmer for 8-10 mins till still firm but cooked through. Check by inserting a knife or skewer into a potato – it should slide through easily. Drain.
2. Meanwhile, rinse the dried porcini mushrooms under cold water and shake off any excess water. Peel and finely chop the onion. Rinse and slice the mushrooms, or tear them into small pieces. Heat your oven to 200°C/Fan 180°C/Gas 6.
3. Set a deep frying pan on a medium heat and add 1 tbsp oil. Add the onion, chestnut mushrooms and the dried porcini mushrooms. Sesaon with a pinch of salt and pepper. Fry, stirring often, for 8-10 mins till the veg are glossy and starting to soften.
4. While the veg fry, peel and grate or crush the garlic. Finely chop the chives. Add the garlic and most of the chives (keep a few pinches back for garnishing) and cook, stirring, for 2 mins.
5. If you haven't already drained the potatoes, check them now and drain them.
6. Add the baby leaf spinach to the pan and cook, stirring, for 3-4 mins till the spinach has wilted. Add the dijon mustard and stir to mix it through. Pour in the double cream and bring to a bubble, then take the pan off the heat. Taste and add more salt and pepper, if you think it needs it.
7. If your frying pan is ovenproof, add the potatoes to the frying pan and gently turn them so they're coated in the cream. If not, arrange the potatoes in an ovenproof dish and spoon over the spinach and cream sauce. Slide into the oven and bake for 20-25 mins till golden brown and bubbling. Serve the gratin garnished with the remaining chopped chives.
8. Love Your Leftovers
The bake will keep in the fridge for up to 3 days. Store it in the dish, covered with food wrap, or transfer to a tub, seal and store. Reheat in the oven (if in the dish) loosely covered with foil or in the microwave. The bake can also be frozen for up to 3 months. Defrost overnight before reheating.