In a large pot bring water to a boil and add a large pinch of salt. Cook the spaghetti to al dente according to the package directions. Drain.
Meanwhile, heat olive oil in a large saute pan over medium heat. Add onions and cook until translucent, 2-3 minutes. Add the garlic and cook 30 seconds longer, until fragrant.
Add grape tomatoes, salt, and red pepper flakes. Cook stirring frequently with a wooden spoon. As they soften, squash the tomatoes with the spoon to release their juice and make a chunky sauce.
Add the cooked spaghetti, about ¾ of the feta, and the olives to the pan and toss to combine.
Garnish with remaining feta and fresh basil and serve.