Bring a pot of salted water to a boil and cook the gnocchi according to package directions, typically 2 to 3 minutes until they float. Drain the gnocchi, reserving about 1/2 cup of the cooking water.

Heat the olive oil in a large, deep skillet or Dutch oven over medium heat. Add the chopped onion and sauté until softened and translucent, about 3 to 4 minutes. Stir in the minced garlic and cook for one more minute until fragrant.

Add the fresh spinach (or squeezed frozen spinach) to the skillet with the aromatics. Cook, stirring constantly, until the spinach has wilted, about 2 to 3 minutes. Season lightly with salt, pepper, and a pinch of dried oregano.

Pour in the heavy cream. Reduce the heat to low and let the cream simmer for about 2 minutes to slightly reduce and thicken. Stir frequently.

Add the grated Parmesan (if using) and at least half of the crumbled feta cheese. Stir until melted and incorporated. If the sauce is too thick, add a splash of reserved gnocchi water. Taste and adjust salt and pepper.

Add the cooked, drained gnocchi into the skillet with the sauce. Toss gently to coat the gnocchi with the sauce. Cook for another minute to heat through.

Transfer to warm bowls. Sprinkle the remaining crumbled feta over the top. Garnish with lemon zest, fresh dill, or parsley if desired. Serve immediately.