Preheat the oven to 375° and lightly grease a baking dish.
Rinse whole mushrooms under cold water and pat them dry with a paper towel. Carefully pop off the mushrooms stems, right where it connects to the cap.
Preheat a saute pan and 1 tbsp of olive oil on medium high heat. Saute diced mushroom stems and diced onion until starting to caramelize. Press garlic into the onion/mushroom mixture and saute just until fragrant. Add cooked veggies into a mixing bowl.
Sift through the crab meat thoroughly with your fingers to make sure there are no pieces of shells accidentally left behind and add crab meat to the mixing bowl.
Add all remaining ingredients for the filling to the mixing bowl and mix very well until all combined.
Set mushroom caps in a slightly greased baking dish, cup up. Generously fill each cap with stuffing mixture, pressing down lightly with your finger to fit a little bit more.
Sprinkle with some grated Parmesan cheese over the top of stuffed mushrooms.
Bake stuffed mushrooms for about 20 minutes (about 25 for larger mushrooms).
GLUTEN FREE NOTE: to make this recipe gluten free, simply use gluten free bread crumbs of the stuffing mixture. Remaining ingredients should already be gluten free.Storing: store leftover mushrooms in an air-tight food storage container, in the refrigerator. Stored properly, mushrooms last 2-3 days. You can reheat individual mushrooms in the microwave for a few seconds but mushrooms don't taste as good. It's best to reheat them in a skillet in the oven, at 350°, just until heated through.