In a bowl, combine the sirloin and 1 1/2 tablespoons soy sauce; set aside.
In a wok or large skillet, bring enough water to reach a depth of 2 inches to a boil. Add the broccoli rabe, lower the heat and simmer for 3 minutes. Drain in a colander and rinse well with cold water; set aside.
Return the wok to high heat, add 2 tablespoons sun-dried-tomato oil and the garlic and stir-fry for 1 minute. Add the sirloin and stir-fry for 2 minutes. Transfer the meat and its juices to a bowl. Add the remaining 1 tablespoon sun-dried tomato oil to the wok, then the scallions, peppers and sun-dried tomatoes; stir-fry for 1 minute.
In a large pot of boiling, salted water, cook the pasta until al dente, about 3 minutes. In a small bowl, stir together the beef broth, cornstarch and remaining 1 1/2 tablespoons soy sauce. Add the mixture to the wok, stirring until thickened, about 1 minute. Return the meat, its juices and the broccoli rabe to the wok; cook until heated through.
Divide the pasta among 4 plates; top with the steak and vegetables.