In a small sauté pan, heat olive oil over medium low heat. Add onions and sauté for 10 to 15 minutes or until translucent and golden in color, stirring occasionally. Season with salt and set aside.
In a small bowl, combine mayonnaise with horseradish and pepper to taste.
Prepare the salad while the onions cook. In a large bowl, combine beans, green onions, herbs, vinegar, olive oil, mustard and Worcestershire. Stir to combine and season to taste with salt and pepper.
To assemble the sandwiches, cut onion rolls in half and spread about 1 tablespoon of horseradish mayonnaise on bottom half of each roll. Divide roast beef evenly between rolls. Top with caramelized onions, sliced apple and a bit of arugula. Serve with White Bean Salad on the side.
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