1. To make the tartare sauce, mix all the ingredients for it in a bowl, then set aside. 
2. Mix the flour, baking powder and all the seasonings together in a large bowl. Place the cornflour in a separate bowl or on a large plate, and place the mushrooms in a third bowl. 
3. Pour a 2cm (1in) depth of oil into a deep frying pan or wok and place over a high heat until it registers 180°C (356°F) on a thermometer. Alternatively, test the temperature by dipping the end of a wooden spoon in it – if it’s hot enough for frying, the oil should bubble around it. Be careful not to overheat the oil or it will begin to smoke. 
4. Whisk the cold beer or soda water into the seasoned flour mixture to make a batter. Then, working quickly, take a piece of oyster mushroom and dip it in the cornflour. Shake off any excess, then dip it in the beer batter before carefully transferring it to the hot oil. Working in batches, cook for 3–4 minutes, or until each piece is golden and crisp. 
5. Once cooked, drain the mushrooms on a wire rack or a plate lined with kitchen paper. Season with extra salt and squeeze over a little fresh lemon. 

For the Potatoes: 

1. Preheat the oven to 200°C/180°C fan/425°F/gas 6.Place a pan of water over a high heat and bring to the boil. Add the potatoes and cook for 15 minutes until tender, then drain. 
2. Lightly grease a baking sheet with a little of the oil, then lay out the potatoes in a single layer. Use a fork to carefully crush and flatten each potato while keeping it in one piece. Scatter over the garlic, drizzle with the remaining oil and season with salt and pepper. Roast for 20–25 minutes until golden brown. 
3. Remove the potatoes from the oven, turn up the heat and quickly place the battered mushrooms on a wire rack over a baking tray and bake for 5 minutes, or until warmed through and crispy. 
4. Serve the mushrooms with the new potatoes, a spoonful of tartare sauce, a squeeze of fresh lemon juice, and a sprinkling of fresh parsley and spring onion.