"This south-of-the-border spin on mac and cheese anchors any game day spread like a strong offensive line and adds real weight to a game day plate. Can garnish with sour cream, hot sauce, more cilantro or more salsa."
Bring a large pot of lightly salted water to a boil. Cook elbow macaroni in the boiling water, stirring occasionally, until cooked through but firm to the bite, 6 to 7 minutes. Drain and transfer to a large mixing bowl; stir in 2 tablespoons butter until coated.
Melt remaining 6 tablespoons butter in a large skillet over medium heat; whisk flour into melted butter. Cook, stirring constantly, until mixture is lightly golden and smooth. Slowly whisk warm milk into flour mixture until smooth. Whisk salt, white pepper, and nutmeg into white sauce, bring to a simmer, and whisk until smooth and thickened, about 3 minutes. Remove sauce from heat and gradually stir Mexican-style cheese into sauce until melted and thick.
Pour cheese sauce over macaroni and gently fold black beans, avocados, green chiles, green onions, and cilantro into the macaroni mixture. Transfer to a 9x13-inch glass baking dish. Spoon salsa over the casserole in decorative swirls or lines.
Bake in the preheated oven until the top has begun to brown and the casserole is bubbling, 30 to 45 minutes. Let the casserole rest about 10 minutes before serving.