Prepare the Beef: Place the thinly sliced beef in a medium bowl. Sprinkle with 2 teaspoons of cornstarch and toss until every piece is lightly coated. Set aside.

Mix the Sauce: In a small bowl, whisk together the soy sauce, beef broth, brown sugar, rice vinegar, and the remaining 1 teaspoon of cornstarch. Set aside.

Sear the Beef: Heat the sesame oil in a large skillet or wok over high heat. Add the beef in a single layer, being careful not to overcrowd the pan (cook in two batches if necessary). Cook for 1-2 minutes per side, until browned but still slightly pink inside. Remove the beef from the skillet and set aside.

Cook the Vegetables: Add the avocado oil to the same skillet. Add the broccoli florets and carrots, stirring frequently for 3-4 minutes until they begin to get tender-crisp. Add the sliced bell pepper, minced garlic, and grated ginger, and cook for another minute until fragrant.

Combine and Thicken: Return the seared beef to the skillet with the vegetables. Give the sauce a final whisk and pour it over everything. Stir continuously until the sauce bubbles and thickens, about 1-2 minutes.

Serve: Remove from the heat and serve immediately. Garnish with sesame seeds and sliced green onions.