Heat the neutral oil in a large, high-sided skillet or wok over medium-high heat. Add the vegetables and cook mostly undisturbed until they're brown around the edges, 8 to 10 minutes, stirring every now and then.
Meanwhile, place the soy sauce, sesame oil, cornstarch, syrup, garlic, ginger, chili flakes, and pepper into a small bowl and whisk to combine.
When the vegetables are ready, pour the sauce into the skillet and stir until the vegetables are evenly coated and glossy, about 30 seconds.