Cut beef thinly across the grain (no more than ½ inch thick, 2 inch long strips). Place it in a freezer bag (or medium bowl). Stir in 2 Tablespoons of soy sauce and the 2 Tablespoons cornstarch and set aside. Allow this to marinate at room temperature for at least 30 minutes.
Prepare the vegetables and set aside.
Prepare the sauce: Whisk together soy sauce, dark soy sauce, brown sugar, garlic, ginger, and sesame seeds. Set this aside.
Heat avocado or vegetable oil to high heat in a large deep skillet or wok.
Add half of the meat to the skillet and fry for 3 minutes. Remove this meat from the skillet and place onto a paper towel lined plate to drain. Fry the other half and place on plate with the other already cooked meat.
Add the vegetables (except the green onions) to the same skillet over medium-high heat. Cook until veggies are tender.
Add the beef back to the skillet.
Stir in the sauce and allow to thicken. Stir and toss to incorporate everything. Sprinkle the green onions and more sesame seeds over top.