Rinse mustard greens thoroughly under cold running water to remove grit. Shake off excess water and tear leaves from stems into roughly 2–3 inch pieces. Thinly slice any tender stems and set aside.

Prepare aromatics and peppers

Thinly slice the onion and garlic, mince the ginger, thinly slice the hot peppers (remove seeds for less heat), and slice the green onions on the bias. Measure out soy sauce, vinegar, sugar, salt, and pepper.

Cook

Heat oil and soften aromatics

In a large skillet or wok, heat 2 tablespoons neutral oil over medium-high heat until hot but not smoking. Add the sliced onion and cook 2–3 minutes until it begins to soften and become translucent. Add the garlic, minced ginger, and sliced hot peppers and cook 30–45 seconds until fragrant, stirring constantly so garlic does not brown.

Add greens and deglaze

Add the torn mustard greens and any tender stems to the skillet. They will mound; stir to start wilting. Pour in 1 cup water or vegetable broth to help steam and deglaze the pan. Use a spatula to scrape any brown bits from the pan.

Season and simmer

Stir in 1 tablespoon soy sauce, 1 teaspoon rice vinegar, and 1 teaspoon sugar. Sprinkle 3 teaspoons kosher salt and 1 teaspoon freshly ground black pepper evenly, then toss to combine. Reduce heat to medium, cover the pan, and let the greens simmer and steam for 4–6 minutes until tender but still bright green. Stir once or twice during cooking to ensure even wilting.

Adjust doneness and seasoning

Uncover, check doneness—if greens are still a bit coarse, cook uncovered 1–2 more minutes. Taste and adjust seasoning: add up to 1 additional teaspoon soy sauce or a pinch more salt if needed. If the pan is dry, add a splash of water.

Finish

Turn off the heat and drizzle 1 teaspoon toasted sesame oil over the greens (optional). Toss in the sliced green onions and give everything a final stir. Transfer to a serving bowl.

Serve

Sprinkle toasted sesame seeds and red pepper flakes if desired. Serve hot as a side dish with rice, grilled meats, or as part of a larger meal.

Leftovers: refrigerate in an airtight container for up to 3 days and gently reheat in a skillet with a splash of water or broth.