Heat the olive oil in a saucepan over medium heat. Add the zucchini and celery and cook, stirring, over low heat for 5 minutes until softened.
Add the garlic, cumin, tomato pasta sauce and ½ cup water. Bring to the boil and season with salt and black pepper. Turn the heat to low, simmer for 10 minutes, add the lentils and cook for a further 5 minutes.
Meanwhile, cook the spaghetti in boiling, salted water according to packet instructions.
Drain the pasta, put into bowls and top with the sauce. Sprinkle with feta and serve.
