Toss shrimp with Cajun seasoning, cover with plastic wrap and refrigerate.
Add flour to oil and cook over medium-low heat, stirring frequently, until mixture turns a deep golden brown color, slightly darker than the color of peanut butter. This will take about 10 minutes. If the mixture is darkening too quickly, reduce heat.
Add shrimp and reserved andouille sausage, stir to combine, then return gumbo to a boil. Cover pot and turn off the heat. Let gumbo sit, covered for 10 minutes before serving. The shrimp will cook perfectly during this 10 minute resting time. Taste and season, if needed. Serve in bowls with basmati rice. Garnish with sliced green (spring) onions.
See Café Tips for more detailed instructions and tips.