Bring a large pot of water to a boil. Cook spaghetti pasta in boiling water until al dente. Drain.
Meanwhile, heat oil in a large frypan or wok over medium-high heat. Cook garlic in oil for 1 minute. Stir in onion and cook until soft. Stir in chicken and cook until juices run clear. Mix in the broccoli, cauliflower and carrots then cook for 2 to 5 minutes, stirring frequently. Season with soy sauce, salt and pepper.