Combine balsamic, mustard, olive oil, shallots, garlic, salt & pepper; whisk to make the marinade. Rinse sirloin, pat dry & place in marinade covered for at least 2 hours.
Heat the grill to medium-high heat. Pat the steak dry & grill 4-6 mins on each side. Baste with the marinade while grilling, reserving ½ C. Remove sirloin from heat & let rest for 5 mins before slicing.
Pour remaining marinade in a small saucepan & bring to a boil, then reduce to a simmer for about 2 minutes, allowing it to thicken. Slice sirloin thinly against the grain & serve with a drizzle of the sauce.