Preheat the oven to 375°F (190°C). Lightly grease a 9-inch pie dish with oil.
Make the dough: In a large bowl, mix the flour and 1 teaspoon salt. Add the warm water and 3 tablespoons olive oil, then stir and knead until a smooth, pliable dough forms (about 2–3 minutes). Cover and let rest for 10 minutes.
Press the dough into the prepared pie dish, covering the bottom and sides in an even layer. If desired, brush lightly with oil.
Cook the filling: Heat 3 tablespoons olive oil in a skillet over medium heat. Add the onion and cook until softened, about 3–4 minutes.
Add the shredded cabbage and cook, stirring occasionally, until tender and lightly golden, 8–10 minutes. Stir in the spinach and cook until wilted, 2–3 minutes. Season with 1 teaspoon salt, black pepper, and garlic powder. Remove from heat and let cool for 5 minutes.
In a bowl, whisk the eggs. Stir in the cooled cabbage mixture until evenly combined.
Pour the filling into the dough-lined dish. Sprinkle the sesame seeds evenly over the top (add extra if you like).
Bake until the center is set and the top is golden, 30–35 minutes. Cool for 5 minutes before slicing and serving warm or at room temperature.