Preparation

• Position a rack in the center of the oven, and heat the oven to 425°F. In a medium bowl, toss the mushrooms with 1/2 cup of the oil. Season well with salt and pepper. Reserving the bowl, transfer the mushrooms to a large rimmed baking sheet, spread in an even layer, and roast for 20 minutes, tossing once, until the mushrooms are fragrant and beginning to brown.
• In the reserved bowl, combine the vinegar and mustard. Whisk well, and then slowly whisk in the remaining 1/2 cup oil.
• Let the mushrooms cool slightly on the baking sheet. Then, using a slotted spoon, transfer them to the bowl. Add the parsley, basil, dill, and arugula. Gently toss to combine, and serve warm or at room temperature.