01 - Bring a large pot of salted water to a boil for cooking the gnocchi.
02 - In a large skillet, heat olive oil and butter over medium heat. Add the chopped onion and sauté until translucent, about 3 minutes. Add the minced garlic and cook for 1 minute more until fragrant.
03 - Add the fresh spinach to the skillet and cook until just wilted. Season with a pinch of salt and pepper.
04 - Pour in the heavy cream and bring to a gentle simmer. Stir in half of the crumbled feta and all of the Parmesan if using. Mix until the cheese is mostly melted into a creamy sauce. Add nutmeg if desired, then reduce heat to low.
05 - Meanwhile, cook the gnocchi according to package directions until they float to the surface, usually 2 to 3 minutes. Drain well.
06 - Add the cooked gnocchi to the skillet and toss to coat in the creamy spinach sauce. Cook for another 1 to 2 minutes, allowing flavors to meld together.
07 - Remove from heat. Top with remaining feta cheese and extra black pepper. Serve warm.