The crawfish butter would also be good served over sautéed fish.
Cook pasta according to package directions, omitting salt and fat. Drain.
Heat a small skillet over medium-high heat. Coat pan with cooking spray. Add onion to pan; sauté 3 minutes. Add Cajun seasoning, red pepper, salt, and garlic to pan; sauté 1 minute, and transfer to a large bowl. Add butter and crawfish to bowl; stir well to combine. Add hot pasta and parsley; toss well. Serve immediately.