Preheat the oven to 375°F (190°C). Line a baking sheet with parchment or lightly oil it.

Clean mushrooms by wiping with a damp paper towel. Twist stems out and set caps aside. Finely chop the stems.

In a small skillet, heat 1 tablespoon olive oil over medium heat. Sauté chopped stems, minced garlic, and most of the green onion (reserve a pinch for garnish) until softened, about 3–4 minutes. Cool slightly.

In a bowl, combine crab meat, softened cream cheese, breadcrumbs, sautéed stem mixture, lemon juice, and 1/4 cup Parmesan. Mix gently so crab pieces stay intact. Taste and adjust salt/pepper.

Spoon the filling into the mushroom caps, mounding slightly. Place them on the prepared baking sheet.

Sprinkle the remaining Parmesan over the filled mushrooms. Drizzle each cap lightly with olive oil.

Bake for 18–22 minutes, until the tops are golden and the mushrooms are tender but not collapsing. For a browned top, switch to broil for the last 1–2 minutes—watch closely.

Remove from the oven and let cool for 3 minutes. Garnish with reserved green onion and a squeeze of lemon if desired.