In a large bowl, mix the ground beef, croutons, sweet onion, egg, garlic, Worcestershire sauce, mustard, red pepper flakes, and Cajun seasoning. Form the mixture by hand into meatballs.
Heat the olive oil and melt the butter in a skillet over medium heat. Place the meatballs in the skillet, and cook, turning constantly, 20 minutes, or to desired doneness.