Heat olive oil in a large skillet over medium-high heat. Sauté the chopped onion for about 5 minutes, stirring occasionally, until softened and slightly browned.
Add the sliced mushrooms, minced garlic, and Italian seasoning to the skillet. Cook for another 5 minutes, stirring occasionally, until the mushrooms release their moisture and begin to brown.
Incorporate the Dijon mustard and pour in the white wine. Allow the mixture to simmer for about a minute, letting the wine reduce slightly.
Pour in the cream and add the uncooked gnocchi. Bring the mixture to a gentle boil, then lower the heat to medium-low and cover the skillet. Cook for 3 minutes.
Uncover the skillet and stir in the fresh spinach. Replace the lid and cook for another 3 minutes, until the spinach is wilted and the gnocchi are tender.
Mix in the grated parmesan cheese and season with salt and pepper to taste. Taste the dish to ensure the gnocchi are cooked through. If needed, cook for an additional few minutes.
Garnish with chopped fresh parsley and serve immediately. For extra indulgence, sprinkle more parmesan cheese on top before serving.