Marinate chicken: In a medium bowl, mix together chicken breasts with rosemary, 3 garlic, olive oil and lemon. Cover and let marinate for 30 minutes.
Cook chicken: Once the chicken is marinated, melt the butter in a large skillet over medium heat. Sprinkle chicken with salt and pepper and sear on both sides, until golden brown, about 2-3 minutes per side. Drain the excess fat from the pan.
Wilt spinach: Preheat the oven to 350 F. Move chicken breasts to one side of the skillet. Stir in chopped spinach to the other side and cook, stirring occasionally, until it begins to wilt, about 1-2 minutes. Set skillet aside.
Make the cream sauce: Melt the butter in a separate skillet over medium heat. Add minced garlic and cook until fragrant, about 1-2 minutes. Whisk in flour and cook for 1-2 minutes. Gradually whisk in milk. Cook, whisking constantly, about 2-3 minutes. Stir in cayenne pepper, nutmeg, rosemary, and grated Grand Cru cheese until slightly thickened. Adjust seasoning with salt and pepper, to taste.
Cook gnocchi: Cook gnocchi according to package directions. Drain in a colander and set aside until casserole assembly.
Assemble the casserole: In the skillet with chicken and spinach, top with cooked gnocchi and cream sauce. Gently stir together. Bake in the oven for 25-30 minutes, or until chicken is cooked through and the sides are bubbling. Serve hot, garnished with parsley if desired.