Marinate Beef: Whisk the marinade ingredients in a large bowl or freezer bag, add the beef, then stir to coat. Let marinate at room temperature for up to 30 minutes or refrigerate for up to 12 hours, then let come to room temperature for 30 minutes before breading.
Sauce: Whisk the ingredients together in a medium bowl; set aside.
Heat Oil: Heat about 1 inch of vegetable or canola oil in a deep enameled cast iron pan (this is what I use) over medium-high heat until it reaches 350°-375°F. (This thermometer comes in handy to monitor oil.) Line a large baking sheet with paper towels. Prep Breading: Add the egg whites to the beef marinade and stir to combine. In a separate large bowl, whisk together the cornstarch and flour.
Cook Beef: Working in batches, dredge the beef strips in the cornstarch/flour until thoroughly coated, then add to the hot oil. Cook for 2-3 minutes, until golden brown and crispy. Transfer the beef to the prepared paper towels. If your beef isn't crispy, your oil isn't hot enough. Repeat until the beef is gone.
Stir-fry Vegetables: Heat 2 tablespoons of neutral oil in a large skillet over medium-high heat. Add the onion and bell pepper and stir fry for 2-3 minutes, until crisp-tender.
Stir-fry Sauce: Add the Sauce and simmer until it thickens to almost the desired consistency. (It will thicken slightly once the beef is added because it is coated in flour/cornstarch).
Combine: Add the beef to the sauce and stir until combined. The longer the beef sits in the sauce, the less crispy (but still delicious) it will become. For the crispiest beef, serve immediately.