Shell bean, sun-dried tomato, and smoked mozzarella salad

6 tablespoons extravirgin olive oil, divided;

1 medium onion, halved lengthwise and cut into thin strips;

1 fennel bulb, cut into thin strips or paper-thin slices;

1⁄4 cup sherry vinegar;

3 garlic cloves, finely chopped;

1 teaspoon Dijon mustard;

2 teaspoons dried oregano;

1⁄2 cup chopped, oil-packed, sundried tomatoes;

8 ounces smoked mozzarella, cut into 1⁄2-inch cubes;

2 tablespoons capers, drained;

Kosher salt and ground black pepper, to taste;

4 cups lightly packed baby arugula or spinach;

Toasted baguette slices topped with olive tapenade, for serving.

Heat 2 tablespoons of the oil in a large skillet over medium heat. Add the onion and fennel and a pinch of salt. Cook, stirring often, until softened and lightly browned, about 15 minutes. Set aside to cool to room temperature.
Whisk together the vinegar, garlic, mustard, and oregano and the remaining 4 tablespoons of oil in a large bowl.
Stir in the beans, tomatoes, mozzarella, and capers. Stir in the onion and fennel. Season with salt and pepper.
Just before serving, stir in the arugula. Serve with tapenade crostini on the side.
Make-ahead note: The salad can be made up to 2 days ahead. Do not stir in the fresh greens until just before serving.