Heat your oven to 200°C/Fan 180°C/Gas 6. Scrub the potatoes and slice them into chips about as thick as your index finger. Pop them in a bowl with 1 tbsp oil and a pinch of salt and pepper. Toss to mix, then spread the chipped potatoes out on a baking tray. Bake in the oven for 30 mins till the potatoes are golden and tender.
While the chips cook, pour the tamari into a dish. Add 2 tbsp vinegar, 1 tsp caster sugar and a pinch of chilli flakes (they're spicy, so use as little or as much as you like). Stir to mix them together.
Peel and grate or crush the garlic. Trim the roots and top 1cm off the spring onions. Finely slice the green parts and set them aside. Finely chop the remaining white parts. Add both the garlic and white spring onion parts to the tamari sauce and stir to mix.
Trim the stalks off the mushrooms so they sit flat. Add the mushrooms to the dish and turn to coat in the marinade, spooning it over them. Set aside to marinate for 10 mins.
Pop the lettuce leaves into a bowl and add 1 tsp olive oil and some salt and pepper. Toss to mix, then set aside.
When the potatoes have 10 mins left to cook, heat a dry frying or griddle pan over a medium-high heat for 2 mins. Lift the mushrooms out of the marinade and place them in the pan. Fry for 3-4 mins, occasionally pressing down with a spatula, till the mushroom are browned and caramelised underneath. Flip them over and fry for another 3-4 mins till the mushrooms are tender.
Pour any remaining marinade into the pan and bubble for 1 min, then take off the heat.
Place the mushrooms and their sauce on 2 warm plates. Serve them with the chips and dressed mixed lettuce leaves.