Grate the veggies. Using the medium or fine side of a box grater, grate the carrot and yellow onion and transfer to a mixing bowl. See below notes on reducing onion flavor, if desired.
Make the dressing. In a mixing bowl combine the mayonnaise, milk, grated onion and carrot, green onion, vinegar, sugar, togarashi, furikake, and salt and pepper to taste. Mix until well-combined.
Cook the pasta. Bring a pot of salted water to a rolling bowl. Add the pasta, stir well, and simmer until just barely past al-dente, but not overcooked.
Drain and rinse the pasta. Drain the pasta in a colander and rinse with cold water until the pasta is cool.
Mix and refrigerate. After rinsing, immediately mix the pasta with the dressing, cover and refrigerate for 1-2 hours, tossing periodically to keep the pasta from drying out. Serve Hawaiian-style mac salad topped with more green onion, togarashi, and furikake, if desired.