Gently wipe 48 large fresh mushrooms with a damp paper towel to clean. Remove the stems by twisting or cutting them off (discard or save for another use, such as stock). Set the mushroom caps aside, cavity side up.
In a skillet over medium heat, cook 1 pound of bulk pork sausage, breaking it into crumbles, until fully cooked and no longer pink, about 7–10 minutes. Drain excess fat and let cool slightly.
In a large bowl, combine 2 lightly beaten eggs, the cooked and crumbled sausage, 1 cup of shredded Swiss cheese, 1/4 cup of mayonnaise, 3 tablespoons of melted butter, 2 tablespoons of finely chopped onion, 2 teaspoons of spicy brown or horseradish mustard, 1 teaspoon of garlic salt, 1 teaspoon of Cajun seasoning, and 1 teaspoon of Worcestershire sauce. Stir until well combined.
Spoon the filling into the cavity of each mushroom cap, mounding it slightly. Use about 1–2 teaspoons of filling per mushroom, depending on size.
Preheat the oven to 350°F (175°C).
Arrange the stuffed mushrooms in two greased 13x9-inch baking dishes.
Bake, uncovered, for 16–20 minutes, until the filling is heated through and the mushrooms are tender.
Transfer the stuffed mushrooms to a platter and serve warm.