Slice the flank steak thinly, about 1/4 inch, against the grain. Place the slices in a medium bowl.

Add 1/4 teaspoon salt and 1 tablespoon of the 3 tablespoons cornstarch to the steak, toss to coat, and let sit 30 minutes.

In a small bowl, whisk together 1/4 cup brown sugar, 2 tablespoons ketchup, 6 tablespoons hoisin sauce, 2 tablespoons low-sodium soy sauce, 1 tablespoon oyster sauce, 2 tablespoons sweet chili sauce, 1 teaspoon crushed red pepper flakes, and 2 tablespoons rice vinegar. Set this sauce mixture aside.

In a separate small bowl, whisk the 1/4 cup cornstarch with the 1/2 cup water until smooth; set this slurry aside.

Heat 1 cup canola oil in a medium-small saucepan or deep skillet over medium-high heat until hot and shimmering. Meanwhile, toss the steak with the remaining 2 tablespoons of the 3 tablespoons cornstarch and shake off any excess.

Fry the steak in small batches (do not overcrowd) for 2–3 minutes per batch, until crisp and browned. Transfer fried beef to a plate lined with paper towels to drain. When finished frying, pour off excess oil from the pan, leaving about 2 tablespoons of oil in the pan.

Heat a large pan or wok over high heat and add the reserved ~2 tablespoons oil. Add the sliced onion and red bell pepper and cook 2–3 minutes, until edges begin to brown. Add the minced garlic and grated ginger and cook 15–30 seconds, until fragrant. Remove the vegetables to a plate and set aside.

Return the pan to high heat, add the prepared sauce mixture, and bring to a simmer. Whisk the cornstarch slurry a little at a time into the simmering sauce until it thickens, about 1–3 minutes.

Add the fried beef and the cooked vegetables to the thickened sauce and toss to coat. Cook 1–2 minutes more, until everything is hot and evenly coated.

Serve immediately.