Erika's Romaine Salad

1 head romaine
1/4 c thinly sliced yellow onion
1/4 c white vinegar
1/4 c water
3 T vegetable oil
1/4 t salt
1/8 t pepper

Chop romaine coarsley. Put in a large bowl with the onion. In a small jar with a lid, shake together the vinegar, water, oil, salt, and pepper. Toss with romaine and onion. Serves 4.