Fill a skillet with 2 inches of water, add salt, then simmer. Meanwhile, cook onion in butter in a nonstick skillet until soft. Add spinach, cream cheese and milk. Cook until creamy. Add salt and pepper. Cover, keep warm.
Toast bread. Break 4 eggs into simmering water. Cook for 1 minute. Loosen from bottom of pan with a spatula. Poach for 3 to 5 minutes, to desired doneness. Remove with a slotted spoon. Top toast with spinach mixture, followed by 2 poached eggs.