Preheat your oven to 375°F (190°C).

Clean the mushrooms with a damp paper towel and gently remove the stems. Set the mushroom caps aside and finely chop the removed stems.

Heat the olive oil in a large skillet over medium heat. Add the chopped mushroom stems and diced onion, cooking until softened, about 5-7 minutes. Add the minced garlic and cook for another minute until fragrant.

In a medium bowl, combine the softened cream cheese, cottage cheese, Greek yogurt, mayonnaise, protein powder, 1/4 cup of the parmesan cheese, mozzarella cheese, black pepper, garlic powder, and salt. Mix until smooth.

Stir the cooked mushroom stem mixture, the well-drained spinach, optional chopped chicken, and toasted nuts into the cheese mixture until everything is evenly combined.

Spoon the filling into each mushroom cap, mounding it slightly.

Place the stuffed mushrooms in a single layer in a baking dish. Sprinkle the remaining 1/4 cup of parmesan cheese over the tops.

Bake for 20-25 minutes, or until the mushrooms are tender and the filling is hot and golden brown.

Serve warm.