In a shallow bowl, whisk together cornstarch, sugar, salt, black pepper, fish sauce, and sauce sauce.
Coat sliced steak in marinade. Cover with plastic wrap and let rest while you prepare the rest of the ingredients.
In a large skillet, heat 1 tablespoon of oil. Add sliced onions. Let cook over medium-high heat for about a minute. Add minced garlic and toss to combine. Let cook for 30-45 seconds until garlic is fragrant. Remove onions and garlic and place in a clean plate.
Add another tablespoon of oil. Add sliced beef in a single layer around hot skillet. Let cook for about 60-90 seconds and then flip beef to cook the other side. If necessary, cook the beef in batches if skillet is not large enough.
Before beef to thoroughly cooked, add onions and garlic back to skillet. Cook until beef is done and onions have softened and cooked.
In a small bowl, whisk together lime juice, rice vinegar, sugar, and water. Allow sugar to dissolve. Add fish sauce and stir to combine. Cover with plastic wrap and let rest while preparing the rest of the ingredients. Add chili garlic sauce option when serving.
In a large bowl, toss arugula and torn mint leaves with a 2-3 tablespoons of prepared salad dressing. Do not drench arugula, just add enough dressing to lightly coat and flavor the arugula.
Divide seasoned arugula among bowls. Add cucumbers, tomatoes, and corn to bowl.
Divide cooked beef and onions among bowls. Serve with lime wedges and additional toppings if desired. Serve with additional dressing. Allow diners to toss and dress their salad with more dressing as desired.
*I prefer to use Viet Huong's Three Crabs fish sauce