Cook the fettuccine pasta according to package instructions until al dente. Drain and set aside, reserving 1/2 cup of the pasta water.
In a large skillet, heat the olive oil over medium-high heat. Add the sliced flank steak and season with salt and pepper. Cook for about 3-4 minutes until browned, then remove from the skillet and set aside.
In the same skillet, add the minced garlic and red pepper flakes. Sauté for about 30 seconds until fragrant.
Stir in the soy sauce, honey, and rice vinegar. Bring the mixture to a simmer, then add the sliced bell peppers and cook for 2-3 minutes until they are tender.
Return the cooked steak to the skillet and mix well. If the sauce is too thick, add some of the reserved pasta water to reach your desired consistency.
Add the cooked fettuccine to the skillet and toss everything together until well combined.
Serve hot, garnished with chopped green onions and grated Parmesan cheese if desired.