Method

• 1

  For the mayonnaise, place the egg yolks into a bowl and whisk together with the vinegar and mustard powder, then season. Rest the bowl on a damp tea towel to stop it moving around and continue to whisk vigorously while slowly dribbling in the rapeseed oil until it thickens. Stir in the tarragon leaves and lemon zest.

• 2

  Preheat a deep fat fryer to 170C/325F. Brush any dirt off the mushrooms, then place the flour and garlic powder into a bowl. In a separate bowl, whisk the eggs together with the milk and put the breadcrumbs into a third. Roll the mushrooms in the flour, then dip into the beaten egg and finally in the dried breadcrumbs until fully coated.

• 3

  Lower the mushrooms into the oil and fry for 3-4 minutes until golden and fully cooked through. Serve immediately with the mayonnaise.