In a large stock pot or Dutch oven, melt ½ cup butter over medium heat. Add onion; cook until translucent, about 7 minutes. Stir in celery, bell peppers, garlic, parsley, and ¾ cup green onion; cook until vegetables are softened, about 10 minutes.
Sprinkle flour over top, and stir for 1 minute. Reduce heat to medium-low and slowly stir in half-and-half. Bring to a boil; reduce heat to low. Simmer until slightly thickened, about 4 minutes.
Slowly stir in American cheese, about ¼ cup at a time, letting each addition melt before adding the next; stir in Monterey Jack cheese in the same manner until incorporated. Stir in Slap Ya Mama Original Blend Cajun Seasoning.
In a large skillet, melt remaining 2 tablespoons butter over medium-high heat. Add shrimp, and sauté until pink and firm, 3 to 4 minutes. Remove from heat.
Combine shrimp, cooked fettuccine, and sauce. Top with parmesan, parsley, and remaining ¼ cup green onion, if desired. Serve immediately.