Cook pasta: Bring a large pot of salted water to a boil and cook 8 ounces of elbow macaroni until al dente, about 7-8 minutes. Drain and set aside.

Sauté aromatics: In a saucepan, melt 2 tablespoons of butter over medium heat. Add diced onion and minced garlic, seasoning with salt and pepper. Cook, stirring frequently, until onions are softened and translucent.

Make roux: Stir in 4 tablespoons of all-purpose flour to the onions and cook for 2-3 minutes, whisking constantly to prevent lumps and remove raw flour taste.

Prepare sauce: Gradually whisk in 2 ½ cups of whole milk, continue stirring until the sauce thickens and bubbles, about 3-5 minutes. Add ¼ teaspoon freshly ground nutmeg.

Add spinach: Stir in fresh or frozen spinach. If using fresh, it may look voluminous but will wilt quickly. Cook until spinach is thoroughly softened and incorporated.

Stir in cheeses: Combine fontina, white cheddar, and Parmesan cheeses (reserve some for topping). Stir most cheeses into the spinach sauce until melted and smooth.

Combine pasta and sauce: Toss the cooked pasta into the cheese sauce, mixing thoroughly to ensure even coating.

Bake: Transfer mixture to a large oven-safe pot or greased 9×13-inch baking dish if skillet is non-oven-safe. Top with the reserved cheese and crispy fried onions or chosen topping evenly.

Finish baking: Bake at 350°F (175°C) for 25-30 minutes until the top is golden and bubbly. For crispier texture, broil for 2-3 minutes at the end.