Preheat the oven to 350 degrees F.
In a large bowl, combine the mayonnaise, mustard, lemon juice, pepper sauce, Worcestershire, salt, and cayenne. Stir until well blended and set aside.
In a medium skillet, melt the butter over medium-high heat. Add the bell peppers, celery, and red onions, and cook, stirring, until wilted, about 2 minutes. Add the green onions and garlic, and cook, stirring, for 30 seconds. Remove from the heat and let cool. When cool, add to the mayonnaise mixture with the parsley and dill, and stir until blended. Gently fold in the crabmeat.
If using the cremini mushrooms, stuff each mushroom cap with about 1 tablespoon of the crabmeat imperial mixture. Place on a baking sheet and sprinkle with Parmesan cheese. Bake until warmed through and bubbly on top, 8 to 12 minutes.
If using the portobello mushrooms, with a paring knife, cut out the gills on the underside of the mushrooms and discard. Lightly oil both sides with olive oil and season lightly with salt and pepper. Place on a baking sheet and bake until tender, about 10 minutes. Remove from the oven and when cool enough to handle, divide the crabmeat imperial mixture among the mushroom caps and sprinkle with the cheese. Bake until the mixture is warmed through and bubbly on top, 12 to 14 minutes.
Remove from the oven and place on a platter. Garnish with the fresh chives and serve.
Yield: 6 to 8 servings (hors d'oeuvre or appetizers)