Cook pasta according to package instructions; set aside.
Place shrimp in a bowl and pat dry with a paper towel to remove any excess moisture. Toss with Cajun seasoning and season with salt and pepper.
Heat half of the olive oil (1 tablespoon) over medium-high heat in a large, deep skillet. Add garlic and heat until fragrant, about 20 seconds, then add peppers and onions. Cook until they begin to caramelize, stirring frequently, about 5-6 minutes; remove from pan and set aside.
Add the remaining 1 tablespoon of olive oil to the pan then add the shrimp in a single layer. Cook shrimp until bright pink, turning once, about 2-3 minutes, being careful not to overcook shrimp.
Stir in the reserved pasta, peppers and onions and toss well. Add additional salt and pepper to taste before serving. Garnish with minced scallions if you wish.