Mix the mustard, soy sauce, flour, 2 Tbsp of the olive oil, 2 tsp of thyme, two of the crushed garlic cloves and 1 Tbsp of cracked black pepper in a small bowl
Coat sirloin with the mustard mixture, put it on a wire rack over a tray and refrigerate for at least 1 hour so the crust sets.
Heat remaining olive oil in a large saucepan, add the onion, carrot, celery and remaining garlic, and cook them for 5 minutes. Pour in the red wine, cook another 5 minutes, then add the beef stock, the bay leaves and remaining thyme. Mix the cornstarch with 1 Tbsp of water until it is smooth and add it to the pan. Simmer the sauce over low heat for 20 minutes or until it is slightly thickened, then strain and season to taste with salt and pepper.
To Freeze: Freeze in freezer safe container, freezing sauce separately.
To Reheat: Defrost. Reheat sauce, add more cornstarch if necessary. Heat grill to medium heat on one side. Place sirloin roast on other side (for indirect heat cooking). Close cover and cook for 45 minutes. Remove from BBQ, cover and let sit 10 mins before slicing. To prepare more quickly, slice sirloin roast before cooking and cook slices to preference. Serve with sauce.