Spread softened butter on Italian bread, set aside.
In a large skillet, heat olive oil on medium-high. Add sirloin slices and generously season with salt and pepper. Sear 2-3 minutes per side. Let rest 5 minutes before slicing.
Heat pan again and saute onion slices and minced garlic. Season with salt and pepper. Stir frequently and cook until brown, 4-5 minutes.
Slice steaks into thin strips while toasting buttered Italian bread in the oven. Add sautéed onions and garlic to steak slices, then place evenly over toasted bread.