Begin with cooking the pasta per box instructions. (Note: Do not overcook the pasta or your noodles will be mushy.) Cut the beef into 2-inch strips and season with salt, pepper and half the garlic.
Add oil to a skillet and heat over medium-high heat. Add the steak and stir fry for one minute, stirring in between. Remove the pieces from the pan and set aside until ready to use.
Add the onions and peppers and cook until the onions are translucent.
Add the oyster sauce, sesame oil, soy sauce, garlic and aji amarillo and stir. Cook for a few seconds.
Add the steak back to the pan and stir. (Note: If you would like a thicker sauce, combine two tablespoons of water and ½ a tablespoon of cornstarch in a separate bowl and stir using a fork. You can now add it to the dish.)
Add the pasta and combine well with the sauce and veggies.
Top the pasta with the scallions and tomatoes and stir well.
Top with some sesame seeds and stir. Cook everything together for another minute or two, stirring in between cooking. The dish is ready to serve.