Prepare the mayonnaise : whisk the egg yolks and add the oil, the garlic adn rosemary mustard and a pinch of salt. Whisk continuously for around 3 to 5 minutes. Store in the fridge.
Heat the oil to 350 F in a deep-fat fryer or large, deep saucepan.
In a large bowl, mix the flour with the beer. Add the egg yolks and the egg white gone up in snow with a pinch of salt.
Dip each fish fillet into the batter.
Carefully lower each fillet into the hot oil. Fry for 4-5 minutes, or until the batter is crisp and golden.
Once cooked, remove the fillets from the hot oil and drain on paper towels. Sprinkle with salt and serve with the homemade mayonnaise and french fries.