Bring a pot of salted water to a boil and cook rotini for 10 minutes until al dente. Drain and rinse under cold water, then drain well.

While the pasta cooks, sear the sliced sausage in a skillet over medium-high heat for 4 to 5 minutes until browned and crisp. Transfer to a plate to cool.

In a large bowl, whisk together mayo, mustard, vinegar, Cajun seasoning, garlic powder, salt, and pepper until smooth.

Add the pasta, sausage, bell peppers, celery, red onion, and most of the green onions to the dressing. Toss for 1 minute until everything is coated. Serve now or chill for later; top with remaining green onions.