Before you start cooking, chop all your vegetables and mince garlic and ginger. I like to have everything ready so I can stir-fry without stopping. It makes the process smooth and less stressful.
Heat vegetable oil in a skillet over medium-high heat. Add the ground beef, breaking it up as it cooks. Once browned, drain any excess fat for a lighter dish.
Add garlic, ginger, broccoli, carrots, and bell pepper to the skillet. Stir-fry for 3–4 minutes until veggies are tender-crisp. I like keeping the veggies slightly crunchy—it adds texture.
In a small bowl, mix soy sauce, brown sugar, rice vinegar, and cornstarch slurry. Pour it over the beef and vegetables, stirring constantly. The sauce will thicken in about a minute, coating everything evenly.
Remove from heat and sprinkle with sliced green onions and sesame seeds. I usually give it a final stir to make sure every bite has some sauce. Serve immediately over rice or noodles.