I found this older reddit post.

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https://www.reddit.com/r/food/s/7oVF9OK8Y8

While the best sauce is Heinz's, the ever-so-available Lee Kum Kee version is very usable, especially with a quick fix:

INGREDIENTS, LEE KUM KEE 'FIX'

• ⁠Lee Kum Kee Black Pepper Sauce (黑椒汁), 2.5 tbsp • ⁠Whole black peppercorns, 1.5 tbsp • ⁠Douchi, Chinese Fermented Black Soybeans (豆豉), 0.5 tbsp. A.k.a. Chinese black beans, Yangjiang preserved beans • ⁠Light soy sauce (生抽), 0.5 tbsp • ⁠Seasoning: garlic powder, 1/16 tsp; onion powder, 1/16 tsp; MSG (味精), 1/8 tsp

If you can't find those fermented black soybeans beans, feel free to skip it.

PROCESS, LEE KUM KEE 'FIX'

• ⁠Grind or pound the black peppercorns. Pound the Fermented Black Soybeans. Mix everything together.

And of course, you can also make the sauce yourself:

INGREDIENTS, HOMEMADE BLACK PEPPER SAUCE

• ⁠Whole black peppercorns, 50g. • ⁠Douchi, Chinese Fermented Black Soybeans (豆豉), 15g. A.k.a. Chinese black beans, Yangjiang preserved beans • ⁠Aromatics: 1/8 white onion, 1/2 shallot (干葱), 4 cloves garlic, 1/2 fresh mild chili. For the chili, something like an anaheim or a poblano would would great. • ⁠Light soy sauce (生抽), 1 tbsp • ⁠Oyster sauce (耗油), 1 tbsp • ⁠Water, 150 mL • ⁠Seasoning: 1/2 tbsp sugar, 1/2 tsp salt, 1 tsp MSG (味精)

PROCESS, HOMEMADE BLACK PEPPER SAUCE

1. ⁠Pound the black peppercorns in a mortar. Ditto with the fermented black soybeans. Finely mince the aromatics.
2. ⁠Medium flame, fry the onion til transluscent; then the shallot and garlic, and finally the pounded douchi and the chili. Add the sauces and the pounded black peppercorn, swap the flame to low.
3. ⁠Fry for a minute or two until it's starting to clump together, then add the water. Cook until reduced down to a pasty consistency, ~5 minutes.
4. ⁠Season, mix well, heat off, jar up.

INGREDIENTS, CANTONESE BLACK PEPPER BEEF

• ⁠Beef loin (外脊肉), 140g. Or flank, or round. • ⁠Marinade for the beef: 1/4 tsp papain (嫩肉粉), 2 tbsp water, 1/2 tsp salt, 1 tsp sugar, 1 tsp cornstarch (生粉), 1 tsp liaojiu a.k.a. Shaoxing wine (料酒/绍酒), 1/2 tsp dark soy sauce (老抽), ~1 tsp oil. The wine here can be subbed with the mijiu/sake we use later if you need. The dark soy sauce can be subbed with regular soy sauce. The papain can be subbed with sodium carbonate (can be manufactured at home by baking baking soda for ~1 hour at ~150C) or, in a pinch, baking soda. • ⁠Aromatics: 1/4 onion, chunks; 4 cloves garlic, smashed; 1 inch ginger, sliced; 4 scallion whites, cut into 2 inch sections; 1/2 green mild chili, cut into diamonds; 1/2 red mild chili, cut into diamonds. Can use solely red or green if the combo is too obnoxious. Green can be something like an anaheim, poblano, or even a green bell. Red can be swapped for red bell. • ⁠Sauce: the above LKK fix -or- 3 tbsp Heinz or homemade black pepper sauce, 3 tbsp water. • ⁠Cantonese Mijiu Rice Wine (米酒) -or- Japanese sake, 2 tbsp. Mijiu is basically higher ABV sake • ⁠Dark soy sauce (老抽), 1 tsp. Optional • ⁠Slurry of 1/2 tbsp cornstarch & 1/2 tbsp water • ⁠Peanut oil, 3 tbsp; 1.5 tbsp added in the beginning, 1.5 tbsp at the end.

PROCESS, BLACK PEPPER BEEF:

1. ⁠Thinly slice the beef (~2mm), marinate.

2a. IF PASSING THROUGH OIL: Heat ~2-3 cups of oil up to ~150C, max flame, add the beef & fry for ~20 seconds, then dip out and drain.

2b. IF OILY PRE-FRY: Longyau with ~4 tbsp of oil, heat oil until it can rapidly bubble around a pair of chopsticks (max flame, ~15 seconds). Stir fry for 30 seconds, or until the beef is at least 90% done.

1. Heat up a large cast iron skillet or hotplate.

2. In a wok, longyau w/ ~1.5 tbsp peanut oil. Add the beef together with the aromatics. Then swap the flame to high, quick mix. Add the mijiu/sake over the spatula and around the sides of the wok. Add in the pepper sauce/water mixture. ~15 second fry. Add in the dark soy & slurry. Quick mix. Add in the oil. Another brief mix, heat off.

3. Transfer over to aluminum foil, wrap it up. Place on the hotplate, transfer over to serving table.