Cook spaghetti according to manufacturer’s instruction in a large pot of salted water, until al dente (about 8 minutes). Drain and put under cold running water to stop the pasta from cooking further. Set aside.
In a wok, heat the oil over medium high heat. Once the oil is hot, add the onion and fry for 2-3 minutes until softened.
Add the chicken fillet and cook for a further 1-11/2 minutes, then add the Tom Yam paste and fry for another minute until fragrant.
Add water and bring to boil. Add the kaffir lime leaves and lemongrass (the white part). Simmer the stew for 2 minutes and Season with fish sauce.
Add prawns and cook for about 1 minute, until cooked through. Then add the straw mushrooms and pasta. Give it a quick stir to coat each strand with the sauce.
Not shown in video : For the roasted cherry tomatoes, preheat oven to 200 degree Celsius. Place tomatoes in a tray, drizzle with some olive oil and place them into preheated oven for 4-5 minutes until the skin looks slightly wrinkled.
To serve: swirl some of the pasta around a fork. Slide the pasta off the fork and mound it in the centre of a plate. Repeat for each serving.
Divide 4-5 prawns per plate, then garnish with kaffir leaves and 2-3 roasted tomatoes per plate. Serve immediately.