Preheat the oven to 400°F and line a baking sheet with parchment paper.
In a medium bowl, combine the panko, pecorino cheese, parsley, sun-dried tomatoes, pine nuts, garlic, salt and several grinds of pepper.
Place the mushrooms, cavity side up, onto the baking sheet. Drizzle the mushrooms with olive oil and sprinkle with salt. Spoon the filling into the mushrooms, drizzle with more olive oil and bake until the mushrooms are tender and the filling is golden brown, 18 to 22 minutes.
Garnish with parsley and sprinkle with red pepper flakes, if desired, and serve.