Thoroughly wash potatoes and carrots, keeping the skins on. Cut off both ends of the carrots. Place the potatoes and carrots in a large saucepan, cover with water and set to boil for 10 minutes or until almost soft.
Carefully remove celeriac skin with a knife, keeping only the clean core. Rinse and cut in half, cover with water. Cook for 5-7 minutes or until almost soft.
Half-fill a small saucepan with water, bring to boil. Add the green peas and cook for the recommended time as per instructions on the packet (usually 2-3minutes).
Once the potatoes, carrots, celeriac and peas are cooked, drain them and set aside. Leave to cool down to room temperature (approximately 1 hr).
In the meantime, finely chop or dice the pickled gherkins, eventually pickled peppers and dice the apple if using.
When the cooked vegetables are cold, peel the skin of the potatoes and carrots and cut into small dices (about 1cm).
Add all prepared veg to a large mixing bowl. Add the dressing: mayonnaise, tartare sauce, dijon mustard and juice from pickles. Season with a pinch of salt and black pepper and toss well so that everything is covered in the dressing.
Cover with a cling film and leave to rest in the fridge for at least 1 hour. Taste, season with more salt and pepper if needed. Serve cold as a side dish or main course.
This salad goes great with roast, grilled, or fried meat. If vegetarian, crumble a boiled egg or two on top and enjoy with a slice of crusty bread.