In a skillet over medium heat, add olive oil. Once hot, add minced garlic and sauté for about 1 minute until fragrant.
Add the chopped spinach to the skillet, cooking until it wilts down, about 3-4 minutes. Season with salt, pepper, and nutmeg. Remove from heat and let it cool slightly.
In a bowl, combine cream cheese, grated parmesan cheese, cooked spinach, and breadcrumbs. Mix well until everything is fully incorporated.
Stuff each portobello mushroom cap generously with the creamy spinach mixture.
Place the filled mushrooms on a baking tray lined with parchment paper.
Bake in the preheated oven for 20-25 minutes, or until the mushrooms are tender and the filling is golden.
Remove from the oven and let cool for a few minutes. Garnish with chopped parsley before serving.