In a 4-quart pot or Dutch oven, melt the butter and olive oil over medium heat. Add the onion and celery and cook for 5 minutes, stirring occasionally, until softened.
Stir in the garlic, Italian seasoning, Dijon mustard, salt, and pepper. Cook for 1 minute, stirring frequently, until fragrant. Sprinkle the flour over the vegetables, stir well and continue cooking for 1-2 minutes.
Slowly stir in the chicken broth, followed by the milk, stirring well so there are no lumps from the flour. Increase heat to medium-high and bring to a boil.
Once boiling, stir in the chicken, gnocchi and carrots. Reduce heat to medium-low and simmer for 5-7 minutes, until the gnocchi is tender and the carrots have softened.
Stir in the spinach and parsley, cooking for 1-2 minutes until the spinach is wilted. Reduce heat to low and stir in the heavy cream, parmesan and lemon juice.
Serve warm with more grated parmesan, chopped parsley and (optional) red pepper flakes.
