Cook the wide rice noodles according to package instructions until al dente, then drain and set aside.
Slice the beef sirloin into thin strips, mince the garlic, slice the red bell pepper, and chop the green onions. Wash the broccoli florets.
In a large skillet or wok, heat the vegetable oil over medium-high heat. Add the thinly sliced beef and stir-fry for 3-4 minutes until browned. Remove the beef and set aside.
In the same skillet, add minced garlic, sliced red bell pepper, and broccoli florets. Stir-fry for 3-5 minutes until tender-crisp.
Return the cooked beef to the skillet and add the drained rice noodles. Mix together soy sauce, oyster sauce, fish sauce, brown sugar, and crushed red pepper flakes, then pour over the noodle mixture. Toss gently until well combined and heated through.
Remove from heat and stir in fresh lime juice. Garnish with fresh basil leaves before serving.