Heat 2 Tbsp. oil in a 10" skillet over medium. Cook onion and leek, stirring occasionally, until very soft but not brown, 10–12 minutes. Transfer to a plate and let cool. Wipe out skillet; set aside.
Whisk eggs, salt, baking powder, pepper, and turmeric in a large bowl. Using a rubber spatula, mix in onion mixture, cilantro, dill, parsley, and fenugreek. (Egg mixture should look thick and very green.)
Heat broiler. Heat remaining 3 Tbsp. oil in reserved skillet over medium. Pour in egg mixture; spread evenly across pan with spatula. Cover and cook frittata until bottom is just set, 8–10 minutes. Uncover and broil, watching carefully, until top is set, about 1 minute. Let cool slightly, then slide out onto a platter.