3 tablespoons red wine vinegar
1 tablespoon olive oil
1/2 teaspoon sugar
1/2 teaspoon salt
1/2 teaspoon prepared mustard
1/4 teaspoon pepper
2 sweet onions, thinly sliced
2 large tomatoes, cut into thin wedges
1/3 cup fresh basil leaves, thinly sliced
Salt and pepper to taste
Whisk together first 6 ingredients in a large bowl; add onions, and toss to coat. Cover and chill at least 8 hours.
Add tomatoes, basil, and salt and pepper to taste to onion mixture, tossing to coat.