Butter a 9x11-inch baking dish.

Combine onion, parsley, and butter in a skillet over medium heat. Cook until onions begin to brown. Pour over bread cubes and toss well.

Whisk together milk, eggs, salt, and pepper. Pour over bread and onion mixture; mix well and allow to stand for 1 hour.

Firmly press mixture into the prepared baking dish; tightly cover with aluminum foil.

Place the baking dish on a rack in a larger pot with 3 inches of water. Cover pot and steam for 1 hour.

Remove the dish from the pot of water and allow to cool for 10 minutes.