This highly flavorful steak is versatile enough to work for family size dining, as well as party-worthy appetizer. It all comes down to the polenta. The dish starts by marinating thin-sliced sirloin in a potent blend of olive oil, balsamic vinegar and garlic. Because the steak is so thinly sliced, it thoroughly absorbs the marinade and cooks almost instantly on the grill. The steak then gets served over the polenta. Ths recipe calls for slicing a tube of prepared polenta into rounds, then brushing them with oil and briefy grilling them. Strips of steak then are piled onto each round and topped with sour cream blended with chives and shallot.