Brush Portobellos with olive oil and grill until done approx 5-7 minutes. Meanwhile saute shrimp, mushrooms, and onions in butter, olive oil and garlic until shrimp turns pink. Remove from heat, add all other ingredients and mix well until mixture becomes firm. Add an additional egg or additional breadcrumbs until the mixture can easily be formed by hand into balls. Place approx 4-5 ounces into portobello mushroom, spreading so that it covers from edge to edge. Sprinkle with shredded mozzarella, and broil until cheese starts to brown. Serve.