Prep the chicken: Place each chicken breast between two sheets of plastic wrap. Pound to an even 1/4–1/2 inch thickness with a meat mallet or rolling pin. Pat dry. Season both sides lightly with salt and pepper.

Make the spinach filling: In a skillet over medium heat, add olive oil. Sauté garlic for 30 seconds until fragrant. Add spinach and a pinch of salt; cook until wilted (or warmed through if using frozen). Stir in red pepper flakes if using. Remove from heat and let cool slightly.

Mix the cheese blend: In a bowl, combine cream cheese, mozzarella, Parmesan, lemon zest, lemon juice, Italian seasoning, onion powder, 1/2 teaspoon salt, 1/2 teaspoon pepper, and Dijon mustard if using. Fold in the cooled spinach until evenly distributed.

Fill and roll: Lay the chicken breasts flat. Spread a generous spoonful of the spinach-cheese mixture onto each, leaving a small border around the edges. Roll up tightly from the short end. Secure with toothpicks or tie with kitchen twine so the filling stays put.

Prep the baking dish: Heat oven to 400°F (200°C). Lightly grease a baking dish. Pour chicken broth or wine into the bottom of the dish to keep things moist.

Arrange and brush: Place chicken roll-ups seam-side down in the dish. Brush tops with melted butter and sprinkle with a pinch of salt and pepper.

Bake: Bake for 22–28 minutes, depending on thickness, until the internal temperature reaches 165°F (74°C). If you want a golden top, broil for 1–2 minutes at the end, watching closely.

Rest and slice: Let the chicken rest for 5 minutes so the juices settle. Remove toothpicks or twine. Slice into rounds or serve whole.

Optional finishing touch: Spoon a little pan juice over the roll-ups and sprinkle with extra Parmesan or fresh parsley.