4 portions. Maximum cost: £8.38, 

Skill level: Easy, 

Total cooking time: 30 minutes, 

  • Prep time: 10 minutes,
  • Cooking time: 20 minutes, 

Equipment used: 1 frying pan, 1 oven proof dish, pestle & mortar, kitchen roll (lots of kitchen roll)


I’m typing this with sticky fingers… buttery, garlicky sticky fingers… (I also have mustard in my hair)

I’ve listed kitchen roll in the equipment list as I like to spread the fried onions out onto a few stacked sheets to remove any excess oil. You can skip this step, but you will still need a ton of kitchen paper to clean up the inevitable mess from your burger. This is a very messy eat. 

And it should be messy – the mushroom is baked in the oven with a ton of garlic butter, topped with halloumi, a big dollop of mayo, some Dijon mustard and a brioche bread roll dipped in the garlic and butter mushroom juices (the vegetarian equivalent of a French Dip). A winning combination.


To prep:

  • Thinly slice 4 small / 3 medium sized onions
  • Dice 4 small garlic cloves into tiny cubes or bash them in a pestle and mortar. To this add 4 large tablespoons of butter and mix – this will top your mushrooms
  • Cut the block of halloumi into 8 pieces


To cook:

Get your onions going. The longer you can cook these for (on the lowest heat possible) the better the end result will be.

Add the sliced onions to a frying pan with a generous drizzle of sunflower oil. If you’re feeling generous, add a dollop of butter as well plus some fresh thyme leaves. Cook on a low heat for a minimum of 20 minutes, although ideally for up to an hour. 

Heat the oven to 180 C. Place the mushrooms in an oven proof dish and top with the garlic butter mixture. Cook for 20 minutes.

When the time is nearly up, scoop your onions to one side of the pan and add the halloumi slices. Cook for a couple of minutes on each side flipping when the cheese starts to turn golden.


To assemble:

Slice the bun in half and spread some good quality mayonnaise onto the base. Add a handful of fresh spinach leaves and the mushroom (you probably want to tip some of the garlic butter into the dish before adding to the burger, but don’t fret, this will not be wasted).

Onto this, add the halloumi, the fried onions and then use the top bun to mop up that garlic butter (everyone loves a soggy bun). Lastly, add some Dijon mustard. 

Now eat, but make sure you have the kitchen roll to hand.



All prices from Sainsbury’s, March 2017, unless otherwise stated

  • 4 Portobello mushrooms (£1.50)
  • 1 pack of halloumi (£2.20)
  • 4 brioche burger buns  (£1.80)
  • 4 small handfuls of spinach (£1 for a 100g bag)
  • 4 small onions (you may have a lot leftover, but better too many than not enough, 68p)
  • 4 cloves of garlic (50p for a bulb)
  • 50g of butter
  • a couple of sprigs of fresh thyme (70p)
  • Dijon mustard


View other vegetarian dishes