Fill a large pot with lightly salted water and bring to a rolling boil over high heat. Once the water is boiling stir in the spaghetti and return to a boil. Cook the pasta uncovered, stirring occasionally until the pasta has cooked through but is still firm to the bite, about 12 minutes. Drain well in a colander set in the sink.
Meanwhile, whisk the cornflour into the vegetable stock until dissolved. Whisk in the peanut butter, soy sauce, honey, brown sugar, sesame oil, ground ginger and red pepper. Bring to a boil over medium-high heat then reduce heat to medium-low and simmer until thickened, about 5 minutes. Stir in the sake and keep warm.
Heat the vegetable oil in a large frypan over medium heat. Stir in the garlic and onion; cook and stir until the onion has softened and turned translucent, about 5 minutes. Stir in the broccoli, carrots, red capsicum and sugar snap peas. Reduce heat, cover and steam until the vegetables are tender, about 5 minutes. Toss the vegetables with the peanut sauce and pasta to serve.