In a Dutch oven, sauté onions, celery, bell peppers, and green onion in butter over medium heat until tender.
Add flour, parsley, and shrimp to the Dutch oven and cook for 15 minutes, stirring occasionally.
Add jalapeño relish, cubed Velveeta cheese, half-and-half, and minced garlic to the mixture.
Reduce heat to low and simmer for 15 minutes, stirring often to prevent sticking and guarantee cheese melts completely.
Season the sauce to taste with Tony Chachere's Creole seasoning.
Meanwhile, cook fettuccine pasta according to package instructions until al dente. Drain and spray with cooking spray to prevent sticking.
Mix the cooked pasta with the creamy shrimp mixture.
Serve immediately, topped with grated parmesan cheese.