Blend the egg yolks, mustard, garlic and lemon juice in a blender. Slowly add the oil, 1 tsp at a time, until you have a mayonnaise consistency. Tilt the blender to the side as you do this to make sure the yolks are in contact with the blades. Add extra water if it’s getting too thick. Season to taste, adding more lemon juice if needed. Place in a bowl and cover until needed.
Heat a large pan with the sunflower oil until it reaches 175°C. Dust the cauliflower in the flour, then dip into the beaten egg, then into the breadcrumbs to coat. Fry in the oil for 5 minutes, turning regularly, until crisp, golden and tender. Remove with a slotted spoon and place on a paper towel. Season lightly.
Heat a griddle pan over a medium-high heat until hot. Add the onions and griddle for 5-7 minutes until cooked, then griddle the halloumi for 2 minutes on each side. Place the buns on the griddle until warm.
Spread the bottom half of the buns with the mayo and place a cauliflower steak on top. Follow with the halloumi, onions and a handful of rocket, then place the bun lid on top.