Place all the sauce ingredients in a small bowl and stir until the cornstarch is dissolved; set aside.
(soy sauce, vinegar, sambal oelek, cornstarch, sesame oil, white sugar)

Prepare veg by cutting cauliflower into 1-inch florets, cutting bell peppers and yellow onion into thin strips, and thinly slicing the green onions

Heat 1 tablespoon of the oil in a 14-inch flat-bottomed wok or large frying pan over medium-high heat until shimmering. Add the cauliflower, season with salt, and spread into an even layer. Cook undisturbed for 1 minute. Stir, spread out the cauliflower again, and cook until starting to brown around the edges, about 1 minute. Add the water, cover, and cook, stirring every minute or so, until the cauliflower is crisp-tender, 4 to 5 minutes. Transfer the cauliflower to a large plate.

Heat the remaining 1 tablespoon oil in the pan until shimmering. Add the bell peppers, onion, and ginger, season with salt, and stir-fry until crisp-tender, 2 to 3 minutes.

Return the cauliflower to the pan. Whisk the reserved sauce to recombine and add to the pan. Cook, tossing constantly, until the sauce thickens and coats the vegetables, 1 to 2 minutes. Sprinkle with the peanuts and scallions and serve.