Preheat the oven to 400° F.
Remove the stems from the mushrooms and clean the mushroom caps with a wet paper towel. Finely chop the stems and set aside.
Place the mushrooms caps on a rimmed baking sheet and brush with a tablespoon of olive oil on all sides. Season to taste with salt and pepper and bake for 15 minutes.
While mushrooms are baking, heat the remaining olive oil and butter in a medium skillet over medium heat.
Add Portobello stems, scallions, and garlic. Season to taste with salt and pepper. Cook, stirring occasionally, until the vegetables become soft and fragrant, approximately 8-10 minutes.
Add chopped shrimp and cook until the shrimp turn pink and opaque, approximately 1-2 minutes.
Remove from heat and stir in the egg, lemon juice, 1/2 of the parsley, and 1/2 of the breadcrumbs.
Remove the Portobello caps from the oven and blot any excess liquid they might have released with a paper towel.
Fill the caps with the shrimp mixture. Top with the remaining breadcrumbs and return into the oven for another 10-15 minutes.
Remove from the oven and serve sprinkled with fresh parsley and extra lemon wedges, if desired. Enjoy!
