1 tablespoon extra-virgin olive oil
1 red onion, thinly sliced
Salt and freshly ground black pepper
1/2 pound shiitake mushrooms, stemmed and sliced 1/2 inch thick
1 (15.5-oz.) can cannellini or white beans, rinsed
1 tablespoon sherry vinegar
1/2 fresh thyme, finely chopped
1/4 teaspoon crushed red pepper flake
5 cups arugula
Dressing
2 tablespoons sherry vinegar
1 tablespoon shallots, finely chopped
1/2 tablespoon Dijon mustard
1/4 cup extra-virgin olive oil