Heat broiler to high. Bring a medium pot of salted water to a boil. Wash and dry all produce. Core, deseed, and dice poblano into ½-inch pieces. Trim and thinly slice scallions, separating whites from greens. Peel and mince garlic. Place smoky red pepper crema in a small bowl. Add water 1 tsp at a time until mixture reaches a drizzling consistency. Season with salt and pepper.
Once water is boiling, add Cavatappi to pot. Cook until al dente, 9-11 minutes. Drain
While pasta cooks, heat a drizzle of oil in a medium, preferably ovenproof, pan over medium heat (use a large pan for 4 servings). Add poblano and season with salt and pepper. Cook, stirring, until softened, 4-5 minutes. Add 3 tablespoons butter, scallion whites, garlic, and Southwest Spice; cook, stirring, until butter melts and scallion whites are softened, 1 minute.
Stir flour into pan and cook until pasty, 1 minute. Gradually stir in ¾ cup water until fully combined. Add cream cheese and cook, stirring, until fully incorporated and sauce has thickened, 1-2 minutes. Turn off heat; stir in cheddar until melted and smooth. Taste and season with salt and pepper.
Stir drained cavatappi into pan with sauce until coated. Taste and season with salt if needed. If mixture is too thick, add a splash more water. TIP: If your pan isn’t ovenproof, transfer mixture now to a baking dish. Using your hands, crush tortilla chips and evenly sprinkle over mac ’n’ cheese. Top with an even layer of Mexican cheese. Broil until cheese melts, 2-3 minutes. TIP: Watch carefully to avoid burning.
Drizzle mac ’n’ cheese with smoky red pepper crema. Top with scallion greens and as much hot sauce as you like. Serve.