Cook the diced veggies in a pan of boiling water for 10 minutes tossing in the frozen peas for the last 3 minutes. Do not cook the onion!
Rinse the cooked veggies under a cold tap and drain thoroughly.
Make the mayonnaise by beating the egg and drizzling in the oil very slowly. Finish off with the garlic and the vinegar and seasoning.
Toss the cold vegetables in the mayonnaise and serve on spoons.