In a small bowl or container combine the vinegar, olive oil, mustard, salt, and pepper.
In a large bowl combine the lettuce and half of the onion, olives, feta, and pear. Gently toss to combine. Transfer to a serving platter, and sprinkle over the rest of the onions, olives, feta and pears. Finish with a bit of salt and and pepper and serve immediately.
Notes
There are only a few things to keep in mind for this to be one of the most delicious and prettiest (but simple) salads you will have this fall. One, slice the onions very, very thinly — it makes such a difference. Two, keep some of the add-ins (onions, olives, pears) to the side when you toss the salad, and sprinkle them over the top, which add a great bit of color at the end.