In a bowl or zip-top bag, combine the sirloin steak with soy sauce, Worcestershire sauce, brown sugar, diced onion, minced garlic, kosher salt, black pepper, and crushed red pepper flakes. Ensure all the meat is well coated in the marinade. Seal the bag or cover the bowl and refrigerate for at least 6 hours or overnight for maximum flavor infusion.

While the beef marinates, bring a pot of cold salted water to a boil. Add the small red new potatoes and let them cook for about 7 minutes, or until they are just tender but still firm enough to hold their shape. Drain the potatoes and allow them to cool slightly before assembling the skewers.

In a mixing bowl, combine the softened unsalted butter with minced Italian parsley, a pinch of black pepper, and a sprinkle of kosher salt. Mix well until all ingredients are thoroughly incorporated. Set this basting butter aside at room temperature.

Preheat your grill to medium heat, around 350°F (175°C). Take skewers (metal is preferred) and start threading the marinated steak pieces and partially cooked potatoes onto the skewers in an alternating fashion. Fill each skewer, leaving a little space between the pieces for even cooking.

Once the grill is ready, place the assembled skewers on the grates. Grill the Marinated Steak and Potato Skewers for about 10 minutes, turning occasionally. Baste with the prepared parsley butter during grilling for added flavor.

Once cooked to your liking, remove the skewers from the grill and let them rest briefly. Serve hot, accompanied by any remaining parsley butter for dipping.