Preheat oven to 350˚F. Butter a 2 quart baking dish. In a large skillet, melt butter and cook the onion for 5 minutes. Add the squash and enough water to barely cover. Bring to a boil. Turn down to a simmer and cook until the squash is tender.
Drain the squash and onion mixture in a colander and gently mash so more juices will drain out. Place in a large mixing bowl. Once drained, add melted butter, eggs, evaporated milk, pimentos, cracker crumbs, cheese, salt, and pepper. Mix well.