Preheat an oven to 350 degrees F (175 degrees C).
Combine the tuna, Swiss cheese, mayonnaise, sweet relish, dried onion, onion powder, salt, and pepper in a large bowl. Fill mushroom caps with the tuna mixture; place on baking sheet.
Bake in preheated oven until the mushrooms are soft and begin to give up their juices, 20 to 25 minutes.
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