Thinly slice the beef into bite-sized strips, then place in a freezer bag. Add the soy sauce, sliced green onions, brown sugar, avocado oil, ginger, and garlic. Mix until all of the meat is coated, then seal and marinate in the fridge for at least 2 hours or up to 24 hours.
Once marinated, remove from the fridge. Heat a large saute pan over medium high heat and add the oil. Add the minced garlic and cook for about 1 minute, then add the broccoli, peppers, salt, and pepper.
Mix and fry until the broccoli is slightly tender and bright green, but still crisp. Remove from the pan and set aside.
In the same pan, add another 1-2 tablespoons of avocado oil. Reserve about 1/2 cup of marinade, then remove the meat. Add half of the meat to the pan. Cook, stirring occasionally, until cooked through—about 2-3 minutes.
Remove the meat from the pan, set aside, and cook the rest of the meat. Set aside.
Add the reserved marinade to your saucepan and bring to a boil for about 2 minutes. Combine the cold water and cornstarch in a small bowl, stirring to dissolve, then pour it into the simmering marinade while whisking. Continue to whisk and simmer until the sauce thickens and coats the back of a spoon.
Add the reserved veggies and meat, then stir together. Serve hot over cooked rice. Enjoy!