Drain the button mushrooms, place them in a large bowl, and pour in the 2 bottles of salad dressing.
Cut the onion into thin slices and then cut the slices in half. Add to the dressing and mushroom mixture.
Cut the peppers into thin rings and then cut the rings into bite-size pieces. Add these to the dressing mixture.
Add the entire bottle of capers (including the juice) into the dressing mixture.
Dice the parsley into small pieces and add to the dressing mixture.
Add the cooked and peeled shrimp to the mixture. Chill and marinate for at least 3 hours before serving.
Notes
To cook the shrimp:
If using frozen shrimp, thaw and rinse. If using fresh shrimp, rinse well.
Bring a large stockpot of water to a rolling boil. Add 2 tablespoons of Old Bay Seasoning. Add the shrimp to the boiling water and continue to cook until the shrimp start rising to the top. Remove from the heat and drain in a colander. Add ice to the top of the shrimp to stop the cooking process. It's very important to not overcook the shrimp! This dish looks very pretty when it is served in a glass trifle bowl.