In a large bowl, combine the ground beef, crushed seasoned croutons, chopped sweet onion, beaten egg, chopped garlic, Worcestershire sauce, yellow mustard, red pepper flakes, and Cajun seasoning. Mix until well incorporated but do not overwork the meat.
Shape the mixture into 1 1/2-inch meatballs and place them on a plate or tray.
Heat the olive oil and butter in a large skillet over medium heat until the butter is melted and starts to foam.
Add the meatballs in a single layer without crowding the pan; you may need to cook in batches.
Cook the meatballs, turning occasionally, until browned on all sides and cooked through, about 15-18 minutes.
Once cooked, remove the meatballs and let rest for a few minutes before serving.
Serve hot as an appetizer, on pasta, or in sandwiches with your favorite sauce.