Combine balsamic, mustard, olive oil, shallots, garlic, salt and pepper; Whisk to make the marinade. Rinse sirloin, pat dry and place in marinade covered for at least 2 hours.
Heat the grill to medium-high heat. Pat the steak dry and grill 4-6 mins on each side. Baste with the marinade while grilling, reserving 1/2 c. Remove sirloin from heat and let rest for 5 mins before slicing.
Pour remaining marinade in a small saucepan and bring to a boil, then reduce to a simmer for about 2 minutes. allowing it to thicken. Slice sirloin thinly against the grain and serve with a drizzle of the sauce.