Melt butter in a 12″ skillet or dutch oven over medium heat. Stir in olive oil and flour. Keep stirring and cooking until you create a medium brown roux. Since this is a small amount of roux, it will brown quickly in about 5 minutes.
Add the onion, celery, and bell pepper, and stir to coat with the roux. Saute vegetables until onion is tender, approximately 5 minutes. Add garlic, and saute one minutes longer.
Add chicken broth a little at a time, stirring constantly to keep the mixture smooth. Add Cajun seasoning and cayenne pepper. Bring to a simmer, and cook for 10 minutes, stirring occasionally.
Stir in tomatoes and pumpkin. Return to a simmer. Cook for at least 10 minutes longer to allow flavors to blend well. (At this point, you could store the mixture in your refrigerator or freezer until you are ready to serve it. Just heat it to a simmer again before going to step 5.)
Stir in shrimp and green onions, and return to a simmer. Cook just until shrimp are opaque and pink. This will only take a few minutes, but it’s hard to say exactly how long since the size of shrimp varies. Watch and stir during this step so you don’t overcook your shrimp!