Steam the mushrooms on a wire rack fitted into a large pot, for about 5 minutes. Remove and place on a cooling rack and allow excess liquid to drain.Cut bacon into a chunky dice and fry until crispy. Add the butter and allow to caramelise and get a deep golden colour. Season well with pepper.Remove the bacon from the pan and cook the spinach in the bacon fat for a few minutes. Season with salt and pepper and allow to cool slightly.In a large mixing bowl add the crispy bacon bits, the cooked spinach, the fresh rocket torn into smaller pieces, the oregano and mix well. Crumble in the feta and give it a final toss.Generously spoon the mixture onto the portabellos and serve straight away.