Preheat oven to 300°.
Arrange edamame in a single layer on a baking sheet; lightly coat with cooking spray.
Combine Cajun seasoning, onion powder, and salt; sprinkle over edamame, and toss to coat. Bake at 300° for 1 hour and 45 minutes, stirring every 30 minutes. Cool completely. Store in an airtight container.