Heat olive oil in a large skillet over medium-high heat. Season the flank steak with salt and pepper, then sear it in the skillet for about 3-4 minutes on each side until browned. Remove the steak from the skillet and set aside.

In the same skillet, add the sliced onion and sauté for about 5 minutes until softened. Add the minced garlic and cook for an additional minute until fragrant.

Pour in the crushed tomatoes, oregano, basil, and red pepper flakes (if using). Stir to combine and bring the mixture to a simmer.

Return the steak to the skillet, spooning some of the sauce over it. Cover and let it simmer for about 20-25 minutes, or until the steak is tender.

Once cooked, remove the steak from the skillet and let it rest for a few minutes before slicing it against the grain. Serve with the sauce spooned over the top and garnish with fresh parsley.