Press the tofu. Wrap in paper towels, weigh down with something heavy, and press for at least 20 minutes.
Cut and coat. Cube the tofu and toss with cornstarch and salt until evenly coated.
Fry. Cook in a single layer in hot oil, 3-4 minutes per side, until deeply golden and crisp. Remove to a paper-towel-lined plate.
Make the sauce. Whisk soy sauce, rice vinegar, brown sugar, and chili garlic sauce. Sauté garlic, ginger, and red pepper flakes in the same pan for 30 seconds.
Thicken. Add the sauce mixture and cornstarch slurry, simmer 1-2 minutes until thickened. Add bell pepper if using.
Toss and serve. Return tofu to the pan, toss quickly to coat, and serve immediately over rice with green onions and sesame seeds.