Preheat the oven to 375°F (190°C).

Clean the Portobello mushrooms and remove the stems.

In a skillet, sauté the minced garlic in a little olive oil until fragrant.

Add the chopped spinach and cook until wilted.

In a mixing bowl, combine the cooked chicken, sautéed spinach, cream cheese, mozzarella cheese, Parmesan cheese, Italian seasoning, salt, and pepper.

Fill each mushroom cap with the chicken and spinach mixture, pressing down gently to pack it in.

Place 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.

Remove from oven and let cool slightly before serving.