In a large skillet over medium-high heat, heat the peanut oil.
Add the minced garlic and minced ginger to the skillet. Stir for 30 seconds.
Add the sliced flank steak to the skillet. Stir-fry for 2 to 3 minutes, until cooked through.
Add the mushrooms, snow peas, shredded carrots and sliced water chestnuts to the skillet.
Stir-fry for 2 to 3 minutes, until the vegetables are tender.
Add the stir-fry sauce to the skillet. Stir until the sauce is heated through.
Add crushed red pepper flakes, if using. Stir to mix.
Transfer the stir fry to a serving bowl.
Sprinkle chopped peanuts and scallions on top, if using.
Notes
Tips and Tricks
Slice the steak - Slice the flank steak across the grain into thin strips so it stays tender.Prep the vegetables - Measure and slice everything before heating the skillet so the timing stays tight.Control the salt - Stir-fry sauce can be salty, so start with 1 tablespoon and add more only if you want it.Add crunch at the end - Add peanuts and scallions after plating so they stay crisp.
Storing and Reheating
Refrigerate - Store leftovers in an airtight container for up to 3 days.Reheat on Stovetop - Place the stir fry in a skillet with 1 to 2 tablespoons water. Warm over medium-low heat, stirring until hot.Reheat in Microwave - Place a serving in a microwave-safe bowl. Cover and heat in 30-second bursts, stirring between rounds, until hot.