Cook fettuccine according to package instructions, drain, and set aside.
Meanwhile, mix soy sauce, brown sugar, rice vinegar, Sriracha, and mustard powder in a small bowl.
In a separate bowl, mix together 2 tablespoons cornstarch and ½ cup water. Set aside.
For the beef, make sure the slices are dry, and season with salt, pepper, and 1 tablespoon cornstarch until well coated.
In a large skillet over medium-high heat add 1 tablespoon of vegetable oil. When pan is hot, add the beef and cook for 2 minutes on each side. Cook in two or three batches, then transfer to a plate.
Add the green beans, onions, garlic, and ginger to the skillet and for 2 minutes.
Pour in sauce and the cornstarch slurry into the pan and stir.
Mix until everything is well coated, then add the beef and toss together with soba noodles.
Transfer to bowls and serve with scallion garnish a squeeze of lime.
Recipe by Ally's Cooking at https://allyscooking.com/2016/07/28/asian-noodles-with-steak-and-green-beans/