Bring a large pot of salted water to a boil. Add the spaghetti and cook until tender but still firm to the bite, stirring occasionally, about 8 minutes Drain
, reserving 1 cup of the cooking liquid.
Meanwhile, heat 2 tablespoons of oil in a heavy large skillet over medium heat. Add the onions and saute until tender and beginning to brown, about 15 minutes adding salt and pepper, to taste. Stir in the marjoram
, and saute until fragrant, about 1 minute. Add the cooked pasta
and the remaining 2 tablespoons of oil and toss with enough reserved cooking liquid, 1/4 cup at a time, to moisten. Toss with the feta cheese. Season the pasta with salt and pepper, to taste. Transfer to bowls and serve.