Open and drain peas. Add chopped onion, tomatoe, salt, and pepper. Do not stir yet. Mix milk with Mayo until blended and pour over peas and vegetables. Mix well and refrigerate if possible for an hour but this is as good served at room temperature, whichever you prefer.
If you'd like, you can also add a little shredded cheddar cheese to taste. Most ingredients are already on hand and this makes it a last minute easy to make side dish for any meal.