Substitute minced beef with ready-made burger patties.
Using your hands, combine the minced beef with the diced red onion, garlic, origanum, paprika, wholegrain mustard and egg and generously season with salt and pepper.
Divide the mince into 4 parts and roll each piece into a ball, then form into flat patties roughly 10cm wide and about 2cm thick.
To help firm up the patties, put them on a plate separating each patty with wax paper, cover and place in the refrigerator until needed.
Remove the stems from the mushrooms and wipe off any remaining soil with a paper towel.
Lightly brush olive oil onto the mushroom caps and season with salt and freshly ground black pepper.
Cook the Portobello mushrooms in a frying pan on a medium-high heat or on a grid over an open fire for about 5 minutes per side or until tender. Alternatively bake them in a 200°C/390°F oven for 15 to 20 minutes.
In the meanwhile, heat a griddle pan on a medium-high heat, brush the burger patties with oil and cook for 3 or 4 minutes on each side or to your liking. Use a spatula to lightly press down on them, making sure each burger is in full contact with the pan.
Build a burger by topping a Portobello mushroom with crunchy lettuce leaves, red onion, fresh tomato and a hot burger patty. Spoon some extra wholegrain mustard over the top of the beef and close the burger with another juicy mushroom. Serve sliced gherkins on the side.