Remove the meatballs from the pan and set aside.
In the same pan, melt some butter and sauté the minced shallots and garlic until fragrant.
Add the heavy cream and Dijon mustard, stirring to combine.
Season with black pepper, salt, and chili flakes.
Add the fresh spinach and cook until wilted.
Stir in the grated Parmesan cheese until melted and the sauce is creamy.
Return the meatballs to the sauce and heat through.
Finish with a squeeze of lemon juice and adjust seasoning if needed.