Preparation

Prepare the beans and mushrooms

• Heat the olive oil in a 12-inch skillet over medium-high heat until shimmering. Add the onions, season generously with salt and pepper, and reduce the heat to medium. Cook, stirring occasionally, until tender and caramel colored, 10 to 12 minutes. Add the shiitake and continue cooking, stirring occasionally, until wilted and soft, about 5 minutes. Off the heat, stir in the beans, vinegar, thyme, and pepper flakes. Season to taste with salt and pepper and let cool to room temperature.

Make the vinaigrette

• In a small bowl, combine the vinegar, shallots, and mustard. Whisk in the olive oil in a steady stream. Season the vinaigrette to taste with salt and pepper.

Assemble the salad

• In a large bowl, gently toss the arugula with enough of the vinaigrette to lightly coat. Season to taste with salt and pepper. Arrange the arugula in a large serving bowl or platter and top with the bean and mushroom mixture and the sliced steak. Drizzle with some of the remaining vinaigrette (you may not need it all) and serve.

Make Ahead Tips

You can make and refrigerate the vinaigrette up to a week ahead, and you can prepare and refrigerate the bean mixture up to a day ahead.

Serve with garlic-rubbed grilled bread.