Roasted Marcona almonds and briny green olives are the perfect cocktail-hour accompaniments to this salad. Serve it with a bright Spanish white or rose, like Verdejo or Rioja rosado.
Place onion in a small bowl of ice water. Let stand 10 minutes, then drain and pat dry.
Using a Microplane, finely grate garlic into a small bowl. Whisk in Dijon and vinegar. Slowly drizzle in oil in a steady stream, whisking until emulsified. Season with salt and pepper.
Arrange tomatoes, pepper, cucumber, and onion on a platter. Toast bread, then cut into 1/2-inch cubes and sprinkle over salad. Season with salt and pepper and drizzle with vinaigrette; serve.