Start by preheating your oven to 375°F (190°C). Clean the fresh mushrooms by gently wiping them with a damp cloth. Remove the stems and set the caps aside. Finely chop the stems, as they will be used in the filling.
In a skillet over medium heat, melt the unsalted butter. Add the finely diced onion along with the chopped mushroom stems. Season with salt and black pepper. Sauté for about 5-7 minutes, or until the onion becomes translucent and tender.
In a mixing bowl, combine the sautéed onion and mushroom stem mixture with the cream cheese and canned chicken. Stir until the ingredients are well incorporated. Finally, fold in the grated mozzarella cheese, ensuring even distribution throughout the mixture.
Take each mushroom cap and fill it generously with the chicken and cheese mixture. Use a spoon to pack the filling in well, forming a small mound on top of each cap.
Arrange the stuffed mushrooms on a baking sheet. Bake in the preheated oven for 20-25 minutes, or until the mushrooms are tender and the filling is golden and bubbly.
Once baked, remove the mushrooms from the oven and let them cool slightly before serving. These Chicken Stuffed Mushrooms are best enjoyed warm, straight from the oven, but they can also be served at room temperature.
