Cook pasta according to package directions until al dente
While pasta is cooking, heat oil in a large skillet and saute onions and bell peppers until tender
Add garlic and sliced andouille sausage and cook for two more minutes
Stir in the sliced green onions and shrimp and cook them for two minutes on each side
Take the skillet off the heat and transfer the mixture to a plate. To make the cheese sauce, melt 2 tbsp butter in a skillet or small saucepan and add the flour. Cook until golden
Add the milk and simmer for 2-3 minutes until thickened, then add 1 cup each Cheddar and Monterey Jack and one tbsp cajun seasoning. Stir until melted.
Mix cheese sauce, pasta and sausage mix together and pour into a 9x13 inch casserole dish
In a small bowl, mix together panko breadcrumbs, 2 tbsp melted butter, remaining sliced green onions, and a pinch of salt
Sprinkle bread crumb mixture evenly over mac and cheese and bake in a preheated 350F for 15 minutes