Preheat oven to 375°F. Line a baking sheet with parchment or grease lightly.
Clean mushroom caps, remove stems, and chop stems finely. Keep the stems to add back into the filling.
Heat olive oil in a skillet and cook stems 3–4 minutes. Stir in spinach, garlic, and artichokes. Cook 2–3 minutes, then remove from heat.
Mix cream cheese, walnuts, parsley, thyme, salt, and pepper in a bowl. Stir in spinach mixture.
Fill mushroom caps with the mixture and place on baking sheet.
Top with Parmesan if desired and bake 15–20 minutes until tender and golden.
Cool slightly before serving. Garnish with fresh parsley if you like.