Boil the spaghetti until slightly firm. You don't want it too soft. While its boiling, fry the onions and garlic in oil. Add salt and pepper and tomato paste. Let everything cook for about a minute then add the freshly cooked spaghetti. Stir to make sure the spaghetti has been totally coated with the tomato mixture. Give it another 3 -5 minutes over medium fire, covered. Stir once more then serve.
This is not exactly low-fat but its quick, easy and veg. I've eaten this through my broke undergraduate days and still whip this up when I'm starving and short on time. Hope you like it!