Teriyaki sauce is a common condiment and easily purchased in bottles, but making your own requires only pantry and refrigerator ingredients. And this way, you can control the fine-tuning of the flavors and texture. This teriyaki sauce is great with meaty, flaky fish and asparagus, mushrooms and/or tofu with leeks, chicken with leeks or scallions, beef and broccoli—on and on. The sauce keeps for weeks in the fridge and is easy to make in large batches if stir-fry is a popular or frequent dinner request.
For the teriyaki sauce: Combine all ingredients except cornstarch in a small pot and bring to low boil, then reduce heat. Whisk together the cornstarch in a few tablespoons water in a small bowl, then add to sauce to thicken to coat spoon. Remove from heat.
For the stir-fry: Place meat in freezer for a few minutes to firm, then trim as needed and halve the tenderloin lengthwise. Press into an even layer and thinly slice meat on bias resulting in thin and uniform strips of pork. Pat meat dry and season.
Heat a large skillet over high heat with oil to coat. When oil smokes and ripples, add pork in 2 batches if necessary to avoid crowding, then cook, stirring occasionally, until browned in places and crispy at edges; remove to plate and repeat if necessary.
Add a bit more oil if pan looks dry and stir-fry the leeks 1 to 2 minutes, then add garlic and peppers and add meat, broccolini, if using, and pour over 2/3 cup teriyaki sauce to combine.
Serve the teriyaki stir-fry with rice and garnishes of choice.