Wipe mushrooms with a clean, damp cloth or paper towel. Remove mushroom stems and chop them coarsely; set aside.
In a large skillet, melt butter or margarine over medium heat. Cook onion, garlic, corn, and chopped mushroom stems in hot butter for 6 to 8 minutes or until tender. Remove from heat. Stir in all remaining ingredients.
Place portobello caps, stem sides up, on a baking sheet. Divide shrimp mixture among portobello caps.
Bake, uncovered, about 15 minutes or until mushrooms are tender.