Part 1: Prepare the Pasta and “Toxic Slime” Purée

1. Preheat and Prepare: Preheat your oven to 375°F (190°C). Lightly grease a 9×13 inch baking dish or a similar 3-quart casserole dish.
2. Cook the Pasta: Bring a large pot of water to a rolling boil. Add a generous amount of salt (it should taste like the sea). Add the cavatappi pasta and cook according to package directions, but undercook it by 1-2 minutes. You want it to be very al dente, as it will continue to cook in the oven. Once cooked, drain the pasta and set it aside. Do not rinse it.
3. Create the Green Purée: While the pasta is cooking, combine all the “Toxic Slime” ingredients (spinach, cilantro, jalapeño, garlic, 1/4 cup milk, and lime juice) in a high-speed blender. Blend on high until the mixture is completely smooth and silky. There should be no visible flecks of spinach left. This might take a minute or two. Set the vibrant green purée aside.

Part 2: Build the Perfect Cheese Sauce

1. Make the Roux: In a large, heavy-bottomed pot or Dutch oven, melt the 6 tablespoons of unsalted butter over medium heat. Once the butter is fully melted and foamy, whisk in the 1/2 cup of all-purpose flour. Cook, whisking constantly, for about 2 minutes. The mixture (roux) should be bubbly and smell slightly nutty. This step is crucial to cook out the raw flour taste.
2. Warm the Milk: While the roux is cooking, gently warm the 4 cups of whole milk in a separate saucepan or in the microwave. Do not let it boil.
3. Create the Béchamel: Gradually pour the warm milk into the roux, about one cup at a time, whisking vigorously and constantly to prevent lumps. Continue whisking until all the milk is incorporated and the sauce is smooth.
4. Thicken the Sauce: Continue to cook the sauce over medium heat, stirring frequently with a wooden spoon or spatula, for about 5-8 minutes. The sauce will thicken enough to coat the back of the spoon.
5. Melt the Cheeses: Turn the heat down to low. Add the softened cream cheese and whisk until it has completely melted into the sauce. Then, add the grated sharp cheddar and Monterey Jack cheese in handfuls, stirring well after each addition until the cheese is fully melted and the sauce is velvety smooth. Important: Do not let the sauce boil after adding the cheese, as this can cause it to become grainy or separate.
6. Season the Sauce: Remove the pot from the heat. Stir in the Dijon mustard, smoked paprika, freshly grated nutmeg, salt, and black pepper. Taste the sauce and adjust the seasoning if necessary. It should be well-seasoned and incredibly flavorful on its own.

Part 3: Assemble and Bake

1. Go Green!: Pour the bright green spinach purée into the cheese sauce. Stir until the color is uniform and a vibrant, “toxic” green. Admire your creation!
2. Combine: Add the cooked al dente pasta to the pot with the green cheese sauce. Gently fold everything together until every piece of pasta is thoroughly coated in the glorious green goo.
3. Prepare the Topping: In a small bowl, combine the panko breadcrumbs, melted butter, and grated Parmesan cheese. Mix with a fork until the breadcrumbs are evenly moistened.
4. Assemble the Casserole: Pour the mac and cheese mixture into your prepared baking dish and spread it into an even layer. Sprinkle the panko topping evenly over the surface.
5. Bake to Perfection: Place the dish in the preheated 375°F (190°C) oven. Bake for 25-30 minutes, or until the sauce is bubbly around the edges and the topping is a deep golden brown and crunchy.
6. Rest: Let the mac and cheese rest for 10-15 minutes before serving. This is a crucial step that allows the sauce to set up, ensuring it’s creamy and not runny.