In a large pan, boil the pasta in salted water for 1 minute less than the packet instructions. When the timer goes off, add the frozen peas to the same pan and cook for a further minute. Drain and reserve the pasta water.
Meanwhile in a bowl, combine the yoghurt, milk, parmesan, lemon juice, lemon zest, black pepper and salt, mix well and set aside for later.
In a medium sized frying pan, add the oil and heat up on medium/high. Fry the sun-dried tomatoes for 2-3 minutes until they have slightly browned, reduce the heat and add the garlic and chilli flakes if using. Fry for another minute then add the drained pasta and peas. Stir well to mix. Remove from the heat, allow to cool for 1 minute then slowly stir through the yoghurt, egg and cheese mix. Return to a very low heat and add in a few splashes (approx 5-8 tbs) of the reserved pasta water to help the sauce emulsify and coat the pasta strands. Check seasoning and serve with some grated parmesan and a sprinkling of chopped parsley.