Cook the spaghetti or linguine according to package instructions until al dente. Drain and set aside, reserving 1/2 cup of pasta water.

In a large skillet over medium-high heat, melt 2 tablespoons of butter. Season the sliced flank steak with salt and pepper, then add it to the skillet. Sear for about 2-3 minutes on each side until browned and cooked through. Remove the steak from the skillet and set aside.

In the same skillet, add the remaining 2 tablespoons of butter and minced garlic. Sauté for about 1 minute until fragrant, being careful not to burn the garlic.

Stir in the soy sauce, Worcestershire sauce, and red pepper flakes (if using). Add the cooked pasta to the skillet, tossing to coat in the sauce. If the noodles seem dry, add a little of the reserved pasta water until you reach your desired consistency.

Return the cooked steak to the skillet and toss everything together until heated through. Adjust seasoning with more salt and pepper if needed.

Serve immediately, garnished with sliced green onions and chopped parsley.