Remove stems from mushrooms; reserve for Seasoning.
Place mushroom caps in baking dish.
Use a plain 1cm pipe to pipe Seasoning into mushroom caps.
Bake in moderately hot oven 10 minutes, top with mayonnaise, garnish with pieces of pepper, if desired; serve immediately.
Seasoning: Cook bacon, celery, shallots and garlic in pan until bacon is crisp.
Add reserved, finely chopped mushroom stems and peppers, cook further 3 minutes.
Place mushroom mixture in bowl, add breadcrumbs, egg yolk, parsley and paprika, mix well.