Melt the I can’t believe it’s not butter in a pan together with the garlic cloves. Place the portobellos on a baking tray and brush with the garlic fat, season with salt and pepper.
Meanwhile, chop the vegetables into small dice and cook in the remaining garlic butter until vegetables are soft. Season with salt and pepper. Stir in the cherry tomatoes, spring onions and basil leaves, cook for a minute or two. Stir in the balsamic vinegar and sugar, remove from heat.Divide the mixture between the two mushrooms, garnish with the olives, sprinkle with the mozzarella and pine nuts. Bake in a moderately hot oven until mozzarella is melted. Serve with freshly picked basil leaves.