Prepare the topping: In a deep saucepan over medium heat, cook breadcrumbs and olive oil until brown. At the end, add the sesame, cook for another 30-60 seconds and transfer to a bowl.
Cook the pasta according to packaging directions.
In the meantime, in the same saucepan you cooked the bread crumbs and over medium/high heat, cook the olive oil, onion and leek until soft, about 10 minutes.
Add the spinach, salt and pepper and cook until wilted, about 5-8 minutes.
Drain the pasta and add it to the saucepan, along with the feta cheese. Stir well to combine and serve on plates.