To make the béarnaise, you’ll need a stick blender and blender cup.
Place the herbs in the blender cup. Blitz with 100ml oil. Carefully crack the egg and place it in the base of the blender cup, taking care not to break the yolk. Pour over the remaining 250ml grape seed oil, add the vinegar, mustard and a good pinch of salt.
Place the stick blender in the blending cup and carefully position the head of the stick blender so it covers and encloses the egg yolk. Blitz and – voilà! – the oil will emulsify with the water in the yolk and the vinegar and the result is a tangy, tarragon heady mayo. Taste and season with a little more salt and or a squeeze of fresh lemon.
Bake or deep fry French fries and serve with béarnaise.