In a medium bowl, combine the water, sugar, ketchup, hoisin, vinegar, soy sauce, chili sauce, sesame oil, vegetable oil, garlic and red pepper flakes. Pour half of it in a zip-top bag. Save the other half for later.
Place the beef in the bag. Massage, seal and refrigerate for at least 6 hours. Make the pickled carrots and sriracha mayo (below).
Just before grilling the meat, place the reserved marinade in a saucepot. Heat over medium heat until it thickens.
Heat the grill to medium-high. Grill steak for 5 minutes.
Flip. Brush with the thickened sauce. Grill 3 more minutes. Flip and coat with more sauce. Grill 2 more minutes.
Remove from grill. Let rest 5-10 minutes. Slice against the grain.