Freeze the flank or flat iron steak for 15-20 minutes (this will make it easy to cut thin slices).
Remove the steak from the freezer, and cut it against the grain into very thin strips. In a very large skillet (preferably a 14-inch diameter so you have room to toss everything later), heat 2 teaspoons of the oil over medium-high heat. Add the steak and sprinkle with the Worcestershire sauce and 1/4 teaspoon of the salt. Cook for 2 to 3 minutes, removing pieces to a plate when they have just a hint of pink left. Wipe out the skillet, and then add the remaining 2 teaspoons oil and sauté the onion over medium-high heat for 3 minutes or until just starting to soften. Add the bell pepper, sprinkle with another 1/4 teaspoon salt and some freshly ground pepper to taste, and stir fry another 4 to 5 minutes or until the vegetables are crisp-tender and the onions are slightly golden.
Meanwhile, cook the pasta according to the package directions in a large pot of salted water. Reserve 1/2 cup of the pasta cooking water. Drain and set aside.
In a separate saucepan or small pot, melt the butter over medium heat. (Watch carefully. Once melted, butter will brown quickly.) Add the flour (or the lesser amount of cornstarch or arrowroot), and whisk for one minute. Slowly whisk in the milk, and then add the salt and pepper. Cook, stirring regularly, until slightly thickened, about 3-5 minutes. Remove from the heat, add the cheese, and stir until melted and smooth.
Add the cooked and drained pasta to the veggies, followed by the steak along with any drippings. Pour the cheese sauce over the pasta mixture, and toss to thoroughly combine. Drizzle with a bit of the pasta cooking water if you prefer a thinner consistency. (I usually just save the pasta water and add a small amount to any leftovers as the pasta will soak up the excess moisture as it sits overnight.)