I chose Walla Walla's.
The marinade is:
1 cup mild olive oil
¼ cup seasoned rice vinegar
2 tablespoons fresh lemon juice
Salt and freshly ground black pepper to taste
2 tablespoons minced fresh basil
2 tablespoons minced fresh parsley
It'll look like this.
Dump the marinade over the onions and chill for 1-2 Hours.
To serve, arrange some tomato wedges in a salad bowl. Layer the onions on top and pour extra marinade over. Easy enough. Goes good with Mozzarella too.