Put 8 cups/2 L of the stock and the wine into a large fish poacher and season to taste with salt. Cover, then place over two burners on the stove. Bring just to a boil over high heat.

Meanwhile, wrap the salmon in several thicknesses of cheesecloth, then tie the ends with kitchen twine. Put the remaining 4 cups/1 L of stock into a small pot and bring to a boil over high heat, for use as needed.

Carefully lower the salmon into the poacher. Add more hot stock, if needed, to cover the salmon entirely, then reduce the heat to medium-low and simmer until the salmon is just cooked through, about 30 minutes. Remove the poacher from the heat and set aside until the salmon and its liquid cool to room temperature.

Carefully remove the salmon from the poacher, holding the cheesecloth by the ends, and transfer to a cutting board, then gently cut off and discard cheesecloth. Keep the skin on, or carefully remove it from the top side of the fish, leaving it intact on the head and tail.

Garnish the salmon with lemon slices, if you like, and serve at room temperature with the mayonnaise.