Heat a wide skillet or wok over medium heat. Add 1 tablespoon butter or neutral oil and swirl to coat. Add the raw cashews in a single layer and toast, stirring frequently, until golden and fragrant, about 4–6 minutes. Transfer toasted cashews to a plate and set aside.

Prepare aromatics and vegetables

Slice the mushrooms, bell pepper, carrot, and scallions. Drain and slice the water chestnuts if not pre-sliced. Mince the garlic and ginger.

Stir-fry vegetables

Wipe the skillet or keep it hot; add 1½ tablespoons vegetable oil and heat until shimmering over medium-high heat. Add the mushrooms and carrot first; stir-fry 2–3 minutes until mushrooms begin to release moisture. Add bell pepper and water chestnuts and stir-fry another 1–2 minutes. Push vegetables to the side of the pan.

Add aromatics

Add the minced garlic and ginger to the empty space in the pan and cook 20–30 seconds until fragrant. Mix with the vegetables and continue to stir-fry for 30–60 seconds.

Combine sauce and finish

Stir the prepared sauce (see Sauce section) to recombine and pour it over the vegetables. Toss to coat and allow the sauce to bubble and thicken about 30–60 seconds. Add toasted cashews and sliced scallions, tossing to combine and warm the cashews through, about 30 seconds. Taste and adjust seasoning with salt, black pepper and optional red pepper flakes.

Sauce

In a small bowl whisk together 2 tablespoons light soy sauce, 1 tablespoon oyster sauce (or vegetarian substitute), 1 teaspoon rice wine or dry sherry, ½ teaspoon sesame oil, and ½ teaspoon sugar or honey until sugar dissolves.

In a separate small cup dissolve ½ teaspoon cornstarch in 2 tablespoons cold water, then stir the slurry into the sauce mixture. Set aside until ready to add to the pan.

Finishing & seasoning

If the stir-fry looks dry after adding the sauce, add up to ½ tablespoon neutral oil. Adjust salt (about ¼ teaspoon kosher salt) and pepper to taste. If you like heat, sprinkle in ¼ teaspoon red pepper flakes and toss to combine. Serve hot.