Remove the stems from the mushrooms with a small sharp knife, then turn the mushrooms over and peel the skin of the caps. Brush the mushrooms all over with oil (leave remaining oil for later) and lightly season with salt and pepper. Place the mushrooms under a grill (broil) on high and grill each side of the mushroom for 5 to 6 minutes or until the mushroom has wilted.
Whilst the mushroom is grilling, prepare the sauce. Place all the sauce ingredients into a food processor or blender and blitz on high for a minute. This sauce will have texture because of the parmesan cheese, so it won't be completely smooth.
Butter the inside of the buns and grill them for approximately 1 minute on each side. Heat oil in a frying pan over medium-low heat, then fry the halloumi on each side for 2-3 minutes or until golden.
Build the burger by starting from the base of the bun. Place the rocket, onions, mushroom, avocado, halloumi and sauce, then close the bun.
Notes
(NOTE 1) Mushroom - Any large mushrooms that are in season will work. I used Portobello because they're large and perfect for burgers. However, if you can't get large mushrooms, grab some medium-sized ones and place 2 of them in each burger.(NOTE 2) Halloumi is a hard Cypriot cheese commonly made with sheep's, goat's or sometimes cow's milk. It's nicknamed 'squeaky cheese' because of its semi-hard and chewy texture. Halloumi is quite salty but is mild tasting. Supermarkets stock halloumi in the chilled section next to other types of cheeses. They package halloumi with some brine, so drain the brine from the halloumi before slicing and cooking.(NOTE 3) Baby rocket (Arugula) - Some people don't like the taste of rocket because they can be bitter. You can swap it with your favourite type of lettuce, such as Iceberg, Oakleaf or even baby spinach.(NOTE 4) Mayonnaise - I love using good quality full fat or regular 'whole egg mayonnaise' for richness and flavour. I find low-fat or 'lite' mayonnaise sweet and artificial tasting.(NOTE 5) Coriander (Cilantro) - There are talks going around that some people think coriander tastes like soap?! I don't get that at all because coriander is one of my favourite herbs! It adds so much flavour to dishes, but if you're not a fan, you could swap coriander with mild-tasting flat-leaf (Continental) parsley.A Third-Party Application calculated the calories and nutritional information. Please use this as an approximate guide only.Cooking measurements are in Australian standard spoon and cup measurements.