Cut the stale croissants up into 1" cubes with a serrated knife and add them to a large bowl.
Add the minced garlic, onion powder, dried thyme, celery seed and salt on top of the cubed croissants. Drizzled with the olive oil.
Toss to ensure the cubed croutons are evenly coated with the olive oil and seasoning.
Pour the seasoned cubed croissants out onto a baking sheet and spread out evenly. Bake for 10 minutes and then use a spatula to turn them over and back for another 5-10 minutes or until they are golden brown and crunchy.
Remove from the oven, let cool and serve or store for later.