Heat the demi glace (or reduce stock until its consistency resembles thin syrup). Season with salt, pepper, and lemon juice.
In a deep fryer or large pot, heat the deep-frying oil to 285°F (140°C). Add the potatoes and fry until light golden brown and almost but not quite cooked through. Strain and transfer to a bowl. Increase the heat to 355 F (180 C).
Just before serving, finish frying the potatoes. In two minutes or less they should be deep golden to light brown and cooked through. Serve the fish with sauce and fries.