Preheat oven to 400°
Clean off mushrooms, I rinse mine and dry them with paper towels. Alternatively, you can wipe them clean with a damp paper towel.
Remove the stems of the mushrooms. (you can scrape the underside of the mushrooms smooth, but I just leave them alone), you can chop the stems to add them to the filling or leave them out, I opted to leave them out and have more crab in the filling.
Remove the crab from the package, chop the crab so when you bite into the mushroom the filling doesn't come out, dice it into 1/4-1/2" pieces.
In a medium bowl, mix the crab, cheese, mayo, red onion and green onion, along with old bay, dill, salt, and pepper.
Place the mushroom on a foil-lined baking sheet (for easy clean up) cap side down. (upside down)
Fill each mushroom with 3 tablespoons of the filling, press it down a bit so it doesn't fall off.
Bake at 400° for 15 minutes or until mushrooms are softened and filling is heated through.
The tops will get golden and crispy.
Serve with lemon squeezed on top (or add some lemon juice to the filling)