1. Heat oven to 400 degrees with racks in upper-middle and lower-middle positions.
2. Toast panko in a large skillet over medium heat, stirring often, until lightly golden, about 3 minutes; transfer to a plate. Add 2 tablespoons oil to skillet and increase heat to medium-high. Season shrimp with salt and pepper, then add to skillet. Cook until just pink, about 1 minute per side, then transfer to a cutting board.
3. Add butter to skillet and cook over medium-high heat until melted and beginning to foam. Add mushroom stems, season with salt and pepper, and cook, stirring frequently, until tender, about 5 minutes. Stir in garlic, lemon zest and juice, and red pepper flakes (if using), and cook 1 minute. Remove from heat and stir in parsley.
4. Finely chop shrimp, then add to mushroom mixture along with panko; season with salt and pepper to taste. Arrange mushroom caps on two parchment-lined baking sheets, then drizzle with remaining 4 tablespoons olive oil and season with salt and pepper. Fill caps with shrimp mixture; they should be quite full, but you may still have a bit of filling left over.
5. Bake until tops are golden, about 25 minutes, rotating baking sheets halfway through. Let cool 5 minutes, then transfer to a platter and serve warm.
Source: Hannaford fresh Magazine, December 2022