Bring a pan of salted water to the boil and cook the spaghetti according to the packet instructions.
While the spaghetti is cooking, spray the frying pan with low calorie cooking spray and set on a medium heat.
Gently fry the onions for 2 minutes until they start to soften, then add garlic, black pepper, thyme and the cherry tomatoes. Cook for another 2 minutes until the tomatoes are beginning to soften.
Add the balsamic vinegar to the tomatoes and continue cooking for a further 2 minutes, then turn down heat and keep warm until spaghetti is cooked.
When spaghetti is cooked, drain and add to the tomato mixture making sure to mix well.
Season with salt and more black pepper if needed.
Sprinkle with the grated Parmesan and serve.