Prep the mushrooms: Heat oven to 375°F (190°C). Wipe mushrooms clean with a damp towel.
Remove stems, chop them finely, and set aside. Lightly oil a baking sheet or line with parchment.
Sauté aromatics: In a skillet, melt 2 tablespoons butter over medium heat. Add chopped stems and shallot.
Cook 3–4 minutes until softened. Stir in garlic and cook 30 seconds. Remove from heat and cool slightly.
Make the filling: In a bowl, combine cream cheese, mayonnaise, Parmesan, 1/4 cup breadcrumbs, parsley, Old Bay, Dijon, lemon zest, and lemon juice.
Mix until smooth. Gently fold in sautéed veggies and crab. Season with salt and pepper.
Fill the caps: Spoon or pipe the filling into each mushroom cap, mounding slightly.
Top for crunch: Mix remaining 1/4 cup breadcrumbs with 1 tablespoon melted butter and a pinch of paprika.
Sprinkle over the stuffed mushrooms.
Bake: Arrange mushrooms on the sheet. Lightly spritz with olive oil for extra browning. Bake 16–20 minutes, until the tops are golden and the filling is hot and bubbly.
Finish and serve: Sprinkle with fresh parsley and a squeeze of lemon.
Serve warm.