Preheat oven to 375 degrees F. Using a sharp knife, cut out the stems from the mushrooms leaving a hole in the center of the mushroom (chop the stems after they have been removed). Place mushroom caps on a baking sheet that is coated with cooking spray.
Melt the butter in a skillet over medium heat. Add chopped mushroom stems, garlic, parsley, salt and pepper. Cook for 3 to 4 minutes, or until mushrooms are soft and garlic is fragrant. Add mixture to a medium bowl, along with crab meat, Parmesan cheese and cayenne pepper (if using). Mix well and set aside.
Cut the cheese into 1/2-inch cubes and place a cube inside each mushroom cap. Top the mushrooms with the crab mixture, dividing evenly. Top with extra Parmesan cheese, if desired, and bake in the preheated oven for 20 to 25 minutes or until cooked through.
Notes
If you are unable to find havarti cheese, replace it with cream cheese.
For an extra bit of flavor and texture, sprinkle sliced green onions over the top just before baking.
Cleaning mushrooms is an important step when making stuffed 'shrooms. Wash them thoroughly with a damp paper towel (make sure all dirt is removed) before diving into the recipe.
Bake as close to serving as possible for the creamiest result.