Preheat your oven to 375°F (190°C).

Bring a large pot of salted water to a boil. Add the elbow macaroni or small shells and cook just until al dente, about 7–8 minutes; drain and set aside.

In a blender combine the baby spinach, parsley, and 1 cup of the milk; blend until completely smooth and a bright green, about 1–2 minutes; set aside.

In a medium saucepan over medium heat, melt 2 tablespoons of unsalted butter. Whisk in 2 tablespoons of all-purpose flour and cook for 1 minute, stirring constantly to form a roux.

Slowly whisk in the blended green milk, then add the remaining 1 cup milk. Cook, whisking continuously, until the sauce thickens slightly and just begins to bubble, about 3–4 minutes.

Reduce heat to low and add the shredded sharp cheddar, shredded mozzarella, and grated Parmesan a handful at a time, stirring until the sauce is smooth and fully melted.

Stir in garlic powder, onion powder, kosher salt, black pepper, and a pinch of ground nutmeg; taste and adjust seasoning. If the sauce is too thick, thin with a splash more milk.

Transfer the drained pasta to a large bowl or back into the pot, pour the green cheese sauce over it, and stir gently to coat every noodle.

Spoon the mac and cheese into an 8×8-inch baking dish and smooth the top.

In a small bowl, mix the panko breadcrumbs with 2 tablespoons melted butter, 1/4 cup finely chopped parsley or chives, 1/4 teaspoon garlic powder, and a pinch of salt; sprinkle evenly over the pasta and press down gently.

Bake 20–25 minutes until the top is golden and the edges are bubbling; for an extra-crisp finish, broil 2–3 minutes at the end—watch closely to avoid burning.

Let the casserole rest 5–10 minutes before serving and garnish with extra parsley or chives if desired.