Place the dried borowik mushrooms in a bowl and cover completely with cold water. Soak for a minimum of 30 minutes, preferably 1 hour or overnight. The mushrooms will soften and the water will turn a deep brown colour. Remove the mushrooms and squeeze gently to remove excess liquid. Slice or roughly chop the rehydrated mushrooms and set aside. Strain the soaking liquid through a fine cloth or coffee filter to remove any grit. Reserve 300ml of this strained liquid. It is intensely flavoured and is the flavour foundation of the sauce.

Melt the butter in a medium saucepan over medium-low heat. Add the finely diced onion and cook gently for 8 to 10 minutes until completely soft and translucent. Add the garlic and cook for a further minute. Add the chopped rehydrated mushrooms and cook for 3 to 4 minutes until they have absorbed the butter and are beginning to colour slightly.

Pour in the reserved mushroom soaking liquid and bring to a gentle simmer. Reduce for 5 to 7 minutes until the liquid has reduced by approximately half and is deeply concentrated. Add the double cream and stir to combine. Simmer gently for a further 5 to 8 minutes until the sauce has thickened to a coating consistency that coats the back of a spoon cleanly. If the sauce becomes too thick, add broth to adjust. Season generously with salt and pepper. Add the fresh parsley or dill off the heat. Keep warm over the lowest possible heat while you make the pancakes.

Peel the potatoes and the onion. Using the largest holes of a box grater, grate the potatoes and onion together into a large bowl. The grating of onion alongside the potato is not decorative. It adds flavour, helps prevent the potato from oxidising and provides a small amount of additional moisture that balances the pressing step.

Now press the liquid from the grated mixture. This is the most critical step in the entire recipe and cannot be abbreviated. Working in batches, take a large handful of the grated mixture and place it in the centre of a clean kitchen cloth or several layers of cheesecloth. Wrap the cloth around the potato mixture and twist and squeeze over the sink with as much force as you can manage. The liquid that comes out will be cloudy and starchy. Continue pressing batch by batch until all the grated mixture has been pressed. The pressed mixture should feel noticeably drier and more compact than raw grated potato. When you think you have pressed enough, press again.

The starchy liquid you have pressed out will separate in the bowl into a clear liquid on top and a white starchy sediment at the bottom. Carefully pour off the clear liquid and reserve the white starch at the bottom of the bowl. This starch goes back into the potato mixture and acts as an additional binder.

In a large bowl combine the pressed potato and onion mixture with the reserved starch, the eggs, flour, salt and pepper. Mix thoroughly. The mixture should hold together when you press a handful of it. If it seems too wet, add another tablespoon of flour. If it seems too dry, it is correct.

Heat enough oil in a wide heavy frying pan to come approximately 5mm up the sides of the pan. The oil must be genuinely hot before the first pancake goes in. Test by dropping a small amount of the potato mixture into the oil. It should sizzle vigorously immediately. If it does not, wait longer.

Take approximately 2 tablespoons of the potato mixture per pancake and place into the hot oil, pressing flat with a spatula to approximately 1cm thickness. Do not crowd the pan. Leave space between each pancake so the oil temperature recovers between additions. Fry without disturbing for 3 to 4 minutes until the underside is deeply golden and a visible crust has formed. Flip carefully and fry for a further 3 to 4 minutes until the second side is equally golden.

Remove to a plate lined with paper towels to drain briefly. Keep warm in a low oven at 200°F while you fry the remaining batches. Maintain the oil temperature throughout by adjusting the heat. The oil must be hot for every batch.

Arrange the placki on warm plates, overlapping slightly. Spoon the warm mushroom cream sauce generously over the top. Add a small amount of sour cream alongside. Scatter fresh chives or dill over everything. Serve immediately. Placki are best eaten the moment they come out of the pan and do not wait well.