Toss the steak, onion, potatoes, olive oil, salt, and pepper together in a large bowl.
Thread onto the skewers, about 9 pieces per skewer.
Melt the butter in a small saucepan on the grill.
Add the garlic and simmer until fragrant, 1 to 2 minutes.
Remove the garlic butter from the grill.
Oil the grill grates.
Place the skewers on the grill, cover, and cook until the beef is seared and cooked to desired doneness, 4 to 5 minutes per side, occasionally brushing both sides of the skewer with the garlic butter.