Move a rack in your oven to the upper third of the oven, then preheat to 425°F (218°F).
Place a large oven-safe pan on the stove over medium-high heat. Pour in the olive oil. Once oil is hot, add diced onion and sliced mushrooms. Sauté, stirring frequently, until onion is transparent and mushrooms are lightly browned, about 5 minutes.
Add garlic and stir for another 30 seconds. Transfer this mixture to a bowl or plate and set aside.
Keeping the heat at medium-high, add the butter in the pan and melt. Once melted, add the flour and whisk until smooth. Continue cooking, whisking constantly for about 1-2 minutes.
Add in chicken bouillon, Italian seasoning, salt, and pepper. Add cayenne pepper if desired.
Very gradually whisk in the chicken broth and then the milk, whisking constantly until the mixture is smooth. Continue to whisk until the mixture thickens, about 5-7 minutes.
Add in the chopped chicken, uncooked gnocchi, mushroom and onion mixture, and coarsely chopped spinach. Stir until everything is coated in cream sauce.
Top with Parmesan cheese and place in the oven. Bake for 18-25 minutes. If desired, broil for 1-2 minutes for a delicious crispy top. Watch closely—it can go from perfect to burnt in a matter of seconds!
Remove from the oven, gently stir, and let stand for about 5-10 minutes to slightly cool and thicken. It’s very creamy right out of the oven but thickens as it stands.
Taste and add any additional salt and pepper as needed. Top with fresh parsley and additional Parmesan if desired.
