Heat a sous vide water bath to 132F degrees.
Salt the London broil on each side (1 teaspoon per side) and let it sit for about 30-90 minutes at room temperature (see note).
Mix the remaining ingredients in a bowl to make the sauce.
Combine the beef and the sauce in a resealable or vacuum seal bag.
Use the "wet" function on the vacuum sealer to seal the bag, or use the water displacement method (above) if using a zip lock bag, to remove all the air.
Place the bag in the heated water bath and cook for 7 hours (add one hour if cooking from frozen).
Remove the bag and place it in an ice bath for 10 minutes.
Remove the beef from the bag and pat dry. Pour the drippings from the bag into a small saucepan.
Bring the sauce to a simmer and cook until its reduced, about 3-4 minutes (see note).
Heat a cast iron skillet or griddle over high heat until smoking and add the London broil.
Sear for about 1 minute per side. Remove, slice, and serve drizzled with the Dijon soy glaze.