Preheat oven to 400°F (200°C). Line a large rimmed baking sheet with parchment paper.

Wipe mushroom caps clean with a damp paper towel. Gently snap off stems and finely dice them. Brush the outside of the hollowed caps with 1 tablespoon olive oil and arrange cavity-side up on the prepared baking sheet.

Melt 1 tablespoon butter and 1 tablespoon olive oil in a skillet over medium heat. Add the minced mushroom stems and minced garlic, sautéing for 4 to 5 minutes until fragrant and moisture evaporates. Add chopped spinach and cook for 1 minute until wilted. Remove from heat and let cool slightly.

In a medium bowl, blend softened cream cheese, grated Parmigiano-Reggiano, chopped parsley, Italian seasoning, sea salt, black pepper, and the cooled sautéed garlic-spinach-stem mixture until completely smooth and creamy.

Spoon generous mounds of the filling into each mushroom cap, smoothing the tops slightly with the back of a spoon.

In a small bowl, toss together panko breadcrumbs, melted butter, and 2 tablespoons parmesan. Sprinkle a generous pinch over each stuffed mushroom cap.

Bake at 400°F for 18 to 22 minutes until mushroom caps are tender and juicy, the filling is piping hot and bubbling, and the panko topping is deeply golden brown. Garnish with chopped parsley and serve warm!