Heat peanut oil in a wok or large frying pan over medium-high heat. Toss the tofu into the oil, and cook until browned on all sides. Once browned, toss in onion, bell pepper, chile pepper, and garlic; cook until just tender, about 5 minutes.
In a small bowl, whisk together the hot water, vinegar, soy sauce, brown sugar, cornstarch, and red pepper flakes. Pour over tofu and vegetables, toss to coat, and simmer 3 to 5 minutes, or until sauce thickens slightly.
