Ingredients: 1/2 cup canola oil, 2 medium red onions (halved lengthwise and thinly sliced), 2/3 cup sherry vinegar, 3 tbsp. freshly squeezed lime juice, 1/2 cup extra-virgin olive oil, 4 medium ripe tomatoes (cut into sixths), 4 medium ripe avocados (halved lengthwise and cut crosswise into sixths), 1 head romaine lettuce torn into bite-size pieces, 1 head Bibb lettuce torn into bite-size pieces, 1 head Boston lettuce torn into bite-size pieces, 1 cup cooked black beans
1. Heat 1/4 cup of the canola oil in a large frying pan over medium heat. When oil shimmers, add onions and cook, stirring occasionally, until golden brown and soft, about 10 minutes.
2. Add 1/3 cup of the sherry vinegar, and scrape up any browned bits that have adhered to the bottom of the pan. Remove from heat and allow to cool.
3. Meanwhile, make the vinaigrette. Combine remaining 1/3 cup sherry vinegar and lime juice in a medium nonreactive bowl, and season with salt and freshly ground black pepper. While constantly whisking, pour in remaining 1/4 cup canola oil and olive oil in a slow, thin stream until dressing is well combined; set aside.
4. To serve, combine cooled onions, tomatoes, avocados, lettuces, and black beans in a large bowl and toss. Add 3/4 of the vinaigrette and toss until greens are well coated. Taste, add more vinaigrette as desired, and season with salt and freshly ground black pepper.
Recipe courtesy of Chow.com.