NOTE — Green Sauce Method:

• Blanch 4 cups baby spinach with 1/2–1 cup basil for 30–45 seconds, then ice-bath and squeeze dry.
• Blend greens with 3/4 cup warm milk until smooth; strain if needed.
• Make mac-and-cheese roux and add warm milk/cream as usual; remove from heat, melt in cheeses, then whisk in green puree gently; avoid simmering after adding puree.

1. Heat oven to 400°F and butter a 9×13-inch baking dish.
2. Boil pasta in salted water to 1 minute shy of al dente; drain well.
3. Warm milk and cream until steamy.
4. In a large pot, melt butter over medium heat; whisk in flour 2–3 minutes to a blond roux.
5. Stream in warm dairy, whisking until smooth and thick enough to coat a spoon, 4–6 minutes.
6. Off heat, whisk in half the cheddar and all Colby Jack (and Gruyère if using) until glossy; stir in sour cream, salt, dry mustard, pepper, nutmeg, Worcestershire, and cayenne; taste and adjust.
7. Fold in pasta; transfer to the dish; top evenly with remaining cheddar.
8. For “Loaded Beast,” scatter bacon, fried shallots, and roasted garlic over the top; reserve green onions for garnish.
9. Bake 25–30 minutes until bubbling; rest 10 minutes; broil 1–2 minutes for deeper crust if desired.
10. Garnish with green onions or cilantro and serve hot.