Preheat the oven to 185°C. In a mixing bowl place the mince, onion, eggs, 4 tablespoons of Simon Gault Italian seasoning and Gault's Beef Stock concentrate then mix thoroughly. Form into 6 large meat patties. In a frypan heat the 50ml olive oil until it starts to shimmer then place the patties into the pan and cook for 5 minutes each side or until your desired degree of cooking. Place the Portobello mushrooms on a baking tray and drizzle with two tablespoon of olive oil and sprinkle with 1 tablespoon of Simon Gault Italian seasoning over all the mushrooms. Place in the oven and cook for 4-5 minutes. While the patties and mushrooms are cooking, slice the burger buns in half and place onto a baking tray. Place the cheese on top of the buns (my favourite is smoked provolone), place into the oven and bake until the cheese is melted, then remove from the oven. On the bun bottom halves spread on the mayonnaise then place the lettuce, tomato, beef patties, a mushroom and top with the top halves of the burger buns. To stop the burger going everywhere when you serve it, use a bamboo skewer and insert it down the middle of the burger and serve.