Spray a frying pan with low calorie cooking spray and place over a medium heat. Add the onion and fry for 5 minutes, stirring until slightly softened and golden brown.
Add the garlic and fry for a further 2 - 3 minutes.
Add the mushrooms and fry for 5 minutes, stirring, over a medium heat.
Add the stock and Worcestershire sauce and stir. Simmer, uncovered, over a medium heat for about 10 minutes or until the stock has reduced by half. Season to taste with salt and pepper and set aside.
Cook the spaghetti in a large pan of boiling water for 9 - 12 minutes, or according to the packet instructions. Drain well and place in a large bowl.
Place the eggs in a bowl and beat with a fork until broken up and evenly mixed.
Stir in the Parmesan cheese and yoghurt and season to taste.
Tip the mushroom mixture and the egg mixture into the bowl of spaghetti and toss to combine well.
Serve immediately, garnished with a little Parmesan cheese and chopped flat leaf parsley.