Gather all ingredients.

Mix ground beef, croutons, sweet onion, egg, garlic, Worcestershire sauce, mustard, red pepper flakes, and Cajun seasoning in a large bowl until well combined.

Form mixture into meatballs by hand.

Melt butter in olive oil in a skillet over medium heat. Place meatballs in the hot skillet. Cook, turning constantly, until desired doneness is reached, about 20 minutes.

An instant-read thermometer inserted into the center should read at least 165 degrees F (74 degrees C) for well done.