Preheat the oven to 400°F. Line a large baking sheet with parchment paper.
In a small bowl, mix together olive oil, garlic powder, onion powder, smoked paprika, dried oregano, salt, and pepper. Rub the spice mixture all over the flank steak.
Place the seasoned flank steak on the prepared baking sheet. Arrange cherry tomatoes, diced cucumber, and sliced red onion around the steak. Drizzle with a little olive oil and season with salt and pepper.
Bake in the preheated oven for 20-25 minutes, or until the steak reaches your desired doneness. Remove from the oven and let it rest for 5 minutes before slicing.
While the steak is resting, prepare the tzatziki sauce. In a medium bowl, combine Greek yogurt, grated cucumber, minced garlic, lemon juice, salt, and pepper. Mix well and set aside.
Slice the flank steak thinly against the grain. Serve the steak in pita bread topped with roasted vegetables, crumbled feta cheese, and a generous dollop of tzatziki sauce. Garnish with fresh parsley.