Slice the beef and put in a bowl with the soy sauce and sherry and set aside to marinate while you prep everything else.
Slice the onions and peppers and the broccoli into bite-sized pieces.
Mince the garlic and ginger and set aside.
Mix the flavoring sauce in one small dish and the cornstarch and broth in another small dish and set aside until needed.
Heat a large skillet or wok over medium high heat. Add 1 tablespoon of the peanut oil and let it get hot. Cook the beef, half at a time, until just cooked through and remove to a bowl. (You can use your serving bowl as the meet is cooked.) This should take about 2-3 minutes, stirring the whole time.
Add the rest of the oil (2 tablespoons) into the pan. Add the onion and stir-fry about 1 minute.
Add the garlic and ginger and stir, then add the broccoli and stir-fry 1-2 minutes.
Add the peppers and stir-fry until all the vegetables are crisp-tender, cooked but still with some crunch, about 1-2 minutes.
Return the beef to the pan.
Stir in the flavoring mix, making sure all the pieces are coated.
Stir the cornstarch and broth mix and add to the pan, stirring, 1-2 minutes. If it's too thick you can add a couple more spoonfuls of broth.
Serve over steamed rice.