Preheat oven to 375°F (190°C).
Gently clean mushroom caps with a damp cloth or paper towel. Remove stems and finely chop them for the filling.
In a skillet over medium heat, sauté the chopped mushroom stems and onion with a pinch of salt until softened (about 4–5 minutes). Let cool slightly.
In a mixing bowl, combine mozzarella, parmesan, sautéed stems/onions, egg whites, coconut flour, garden blend seasoning, parsley, and remaining salt. Stir until well mixed. The egg whites and coconut flour will help bind the mixture.
Place mushroom caps on a parchment-lined baking sheet.
Spoon filling generously into each cap, pressing slightly to mound.
Bake for 18–20 minutes, until the cheese is melted, tops are golden, and mushrooms are tender.
Garnish with a little extra parsley if desired.
