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.
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.
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.
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.
Beverage pairing: Veramonte Sauvignon Blanc Reserva, Chile. The clean, bright flavors and contrasting creamy/crunchy textures of the salad need the kind of do-it-all white wine that’s perfect for summer afternoon sipping. This inexpensive but well-made Sauvignon Blanc from Chile is that wine. With citrus, herbs, flowers, and some tanginess, it’s almost a salad dressing unto itself.
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