Preheat oven to 425 degree F. Wipe mushrooms with a clean, damp cloth or rinse lightly and dry gently with paper towels. Cut mushroom stems flush with caps. Chop enough of the stems to make 1/2 cup; set aside.
In a large skillet, melt butter or margarine over medium heat. Cook onion, garlic, and chopped mushroom stems in hot butter for 6 to 8 minutes or until tender. Cool slightly. Stir in chopped shrimp, bread crumbs, egg, lemon juice, and tarragon.
Place portobello caps, stem sides up, in a 15x10x1-inch baking pan. Sprinkle mushrooms with cheese, if desired. Divide shrimp mixture among portobello caps. Bake, uncovered, about 15 minutes or until mushrooms are tender. To serve, garnish with additional whole shrimp and parsley, if desired. Cut into quarters, if desired. Makes 4 appetizer servings.