Heat the oven to 220C (200C fan)/425F/gas 7. Put the mushrooms, potatoes, chilli, five tablespoons of oil and the lemon peel in a 35cm x 25cm roasting tray, season with a quarter teaspoon of salt and a good grind of pepper, and roast for 35 minutes, stirring twice, so the potatoes and mushrooms don’t stick to the tray, until soft and golden brown.
Meanwhile, make the meatballs. Drain the porcini, squeezing out as much water as possible, then finely chop. Put in a large bowl with all the other meatball ingredients, three-quarters of a teaspoon of salt and plenty of pepper. Mix until well combined, then form into eight large, 110g or so, meatballs, squeezing to compact them.
Pour the stock over the potatoes and mushrooms, arrange the meatballs evenly around the tray and return to the oven for 35 minutes, turning the tray around once halfway through the roasting, until the meatballs are cooked through and golden-brown.
While the meatballs are cooking, mix the celery, celery leaves, currants and parsley in a medium-sized bowl with the lemon juice, the remaining 25ml olive oil, a quarter-teaspoon of salt and a good grind of pepper.
When you’re ready to eat, top the meatballs with the celery mixture and serve directly from the tray.