1.  Prep the stir-fry ingredients

• Peel and thinly slice the red onion.
• Trim the root ends from the scallions; cut the scallions into 1-inch lengths.
• Finely chop, press, or grate enough garlic to measure 1 teaspoon.

2.  Sear the steaks

• Pat the steaks dry with a paper towel. Cut the steaks into 1-inch pieces; season generously with salt and pepper.

In a large frying pan over medium-high heat, warm 1 tablespoon oil until hot but not smoking. Add the steak; cook, stirring occasionally, until browned but not yet cooked through, 2 to 4 minutes. Transfer to a plate. Do not wipe out the pan.
While the steaks cook, prepare the watercress and make the lime-pepper dipping sauce.

3.  Prep the watercress and lime-pepper dipping sauce

• Trim the root ends and any coarse stems from the watercress.
• Juice the lime.

In a small bowl, stir together 2 tablespoons lime juice and half the salt and pepper blend; add more of the blend to taste.

4.  Finish the stir-fry

In the same pan used for the steak, if dry, add 1 tablespoon oil; warm over medium-high heat until hot but not smoking. Add the onion, season with salt, and cook, stirring occasionally, until softened, 3 to 4 minutes. Stir in the scallions and garlic; cook until the scallions soften, 1 to 2 minutes.
Add the steak and any accumulated juices. Stir in the stir-fry seasoning base and the mirin or 2 tablespoons white wine or water; cook, stirring to coat, for 1 minute. Remove from the heat; stir in 1 tablespoon butter, if using.

5.  Serve

Transfer the watercress to individual plates and top with the stir-fry. Serve with the sweet potatoes and lime-pepper dipping sauce. 

NOTE:  Roasted sweet potatoes go well with this dish.