Preheat oven to 400 degrees
Prep large baking dish with nonstick baking spray.
Whisk to combine the Crawfish boil seasoning, cajun spice, and garlic powder in a bowl. Put red potatoes and sliced onions in the prepared baking dish.
Season with 1⁄2 of the spice mix.
Bake at 400 degrees for 15 minutes.
Remove from oven.
Add the shrimp and sausage.
Season with the remaining 1⁄2 cup of spice mix.
Mix to combine.
Return to the oven at 400 degrees for 10 minutes.