Remove stems from mushroom caps and dice stems, set aside.
Carefully scrape away the dark gills from the bottoms of the mushroom caps and discard.
Melt butter in a large skillet over medium heat.
Add shrimp, onion, chopped stems and bell pepper to skillet, sauteing until shrimp is pink and other ingredients are softened, being careful not to overcook the shrimp (shrimp will get rubbery if overcooked).
Remove mixture from heat and place in a large mixing bowl (do not drain liquid).
Add cream cheese to mixture and stir well.
Then add all other ingredients, except mushroom caps and one half cup shredded cheddar cheese, and mix well.
Spray an 8x11 glass baking dish with non-stick cooking spray.
Carefully stuff the mushroom caps with stuffing mixture and place into baking dish.
Evenly spread remaining half cup of cheddar cheese over stuffed caps.
Place dish into preheated oven, and bake for apx. 45 minutes to one hour.