Preheat oven to 450 F degrees and set to broiler. Set your rack in the middle.
Season the flank steak with salt and pepper and chili powder for a bit of a kick. Add a tbsp of olive oil to an oven proof skillet.Place flank steak in the skillet and place the skillet in the oven. Once the top side is crispy to your liking flip it over and broil for another 7 minutes or so. Remove from oven, cover in aluminum foil and let it rest for 15 minutes.
In the mean time, saute the onion with the bell peppers in the remaining 2 tbsp of olive oil. Season with the Tex Mex seasoning and add additional salt and pepper if needed. Saute until onion is caramelized.
Slice the flank steak into thin slices. Assemble your tostadas by layering onion and peppers, steak, then top with tomatoes, feta cheese and garnish with parsley.