Heat grill or broiler. In resealable plastic food storage bag or glass dish, combine red wine and sirloin; seal bag. Marinate steak at room temperature for 15 minutes. Remove sirloin from bag. Season on both sides with salt and pepper.Sauce:
In a small saucepan, whisk together wine, broth, mustard, sugar, onion powder, salt and pepper. Bring to simmering over medium-high; simmer until reduced to about 1-1/4 cups, 20 minutes. Grill or broil sirloin for 3 to 5 minutes, depending on thickness. Flip steak over; grill another 3 to 5 minutes or until internal temperature registers 145 degrees on instant read-thermometer for medium rare. Transfer to a cutting board. Let stand 5 minutes.
Stir flour-water mixture into red wine sauce; simmer until thickened, about 3 minutes.
Cut sirloin across grain, into thin slices. Drizzle with red wine sauce. Serve warm. Makes 4 servings.