Wash the portabello mushrooms, remove the stalks and place on baking tray with two tablespoons of water. Bake in the over at 180 degrees celcius/356 f for 10 minutes. While they're baking make the stuffing as follows.
Fry the onion in an inch of water for three minutes, add more water if it evaporates.
Add the garlic and chopped mushroom stalks and cook for a further two minutes.
Add the breadcrumbs, olives, mixed nuts, parsley, red pepper, oregano and black pepper. Stir until all the ingredients are mixed together, and hold well without falling apart. You may need to add more water to help the mixture stick together.
Remove the mushrooms from the oven and place slices of tomatoes on the gills of the mushrooms, then divide the mixture equally between the four mushrooms. Adding the chopped spring onions/scallions to the top.
Return the mushrooms to the oven, add more water if it's evaporated to the baking tray, and bake for a further 10-15 minutes. You want the mixture to have a nice crunchy top.
Add the chives then serve. I served mine on a bed of baby gem lettuce with chopped tomatoes, chives and avocado.