Combine all the ingredients in a bowl, stirring until the sugar dissolves.
Bring the seasoned pho stock to the boil in a saucepan. Slowly add the slurry in a thin stream, whisking constantly. Stop adding the slurry when you reach an unctuous, saucy consistency (you will not need to add all the starch). Cover, set aside and keep warm.
Mix all the ingredients together in a bowl until well combined, then season to taste with salt and pepper.
Season and grill the steaks in a frying pan or on a chargrill on high to your preferred degree of doneness (rare to medium is best in my opinion). Transfer to a plate, cover with foil and set aside to rest for 5 minutes.