Preheat your oven to 400°F. Lightly coat a large rimmed baking sheet or baking dish with non-stick cooking spray and set it aside.

Clean the mushrooms gently with a damp paper towel to remove any loose dirt. Carefully pop the stems out of each mushroom cap. Set the hollow caps aside and finely chop the removed stems to use in the filling baseline.

Melt the 2 tablespoons of unsalted butter in a medium skillet set over medium heat. Add the minced garlic and the finely chopped mushroom stems, sautéing for 4 to 5 minutes until the vegetables soften and release their moisture. Remove the pan from the heat.

In a medium mixing bowl, combine the softened cream cheese, grated parmesan cheese, shredded cheddar, sliced green onions, Old Bay seasoning, lemon juice, and the sautéed garlic-stem mixture. Whisk firmly with a fork until the base forms a smooth, cohesive matrix.

Gently fold the drained lump crabmeat and exactly ¼ cup of the panko breadcrumbs into the cream cheese mixture. Use a silicone spatula to combine the elements carefully so you keep the sweet crab chunks relatively intact. Season with a tiny pinch of salt and pepper to preference.

Arrange the hollowed-out mushroom caps gill-side up on your prepared baking sheet.

Spoon a generous portion of the crab filling into each cap, pressing down lightly to secure it and mounding it slightly over the top edges.

Scatter the remaining ¼ cup of crisp panko breadcrumbs uniformly over the top of the stuffed mounds to create a texturized upper layer.

Place the baking sheet on the middle oven rack and bake at 400°F for 18 to 20 minutes. The appetizers are finished when the mushroom bodies are tender and juicy, and the top breadcrumbs take on a beautiful golden-brown color.

Remove the pan from the oven and let the stuffed mushrooms rest flat on a wire cooling rack for 5 minutes. This brief pause allows the hot cheese emulsion to tighten slightly, making them easy to handle. Garnish with a sprinkle of fresh chopped parsley and serve warm.