Heat a skillet over medium heat, melt the butter, and sauté the garlic and onion until the onion is translucent. Add the green bell pepper and cook for 1 minute. Stir in the flour and cook until lightly golden.

Gradually whisk in the milk to dissolve the flour, and cook slowly for about 5 minutes, making sure the flour doesn't burn. Add the crab meat, parsley, pepper, and salt, and mix well. Preheat the oven to broil.

Stuff the mushrooms with the crab mixture, sprinkle with breadcrumbs, and spray with oil. Place the mushrooms on a baking sheet, put them in the oven, and broil until golden. The mushrooms should be just tender, not overcooked.