In a mixing bowl, combine the gochujang, Thai red curry paste, tamari or soy sauce, fish sauce (if using), and pickled ginger juice. Whisk together until smooth and set aside.
Heat a wok or large skillet over medium-high heat. Add the sesame oil or olive oil, swirling to coat the pan.
Add the thinly sliced yellow onion to the hot wok. Stir-fry for about 2 minutes until it becomes translucent.
Increase the heat to high and add the thinly sliced flank steak. Stir-fry for 2-3 minutes until the beef is browned and cooked through.
Add the mixed chopped vegetables to the wok. Stir-fry for another 3-4 minutes until the vegetables are tender yet crisp.
Pour the prepared sauce over the beef and vegetables. Toss everything together to ensure the sauce evenly coats the mixture.
Remove the wok from heat and stir in the fresh basil and sesame seeds. Add chili flakes to taste for an extra kick.
Serve the stir fry hot, either on its own or over a bed of rice or noodles. Enjoy your delicious 20 Minute Sesame Basil Beef Stir Fry!