Lamb Salad

Ingredients

• 1 head garlic
• 2 tbsp. canola oil
• 2 tbsp. sherry vinegar
• 2 tbsp. salt-packed capers, rinsed, drained, and finely chopped
• 2 tbsp. minced shallots
• 10 oil-packed anchovy filets, drained and finely chopped
• 12 tbsp. extra-virgin olive oil
• Kosher salt
• freshly ground black pepper
• 1 (1 1/2-lb.) lamb top round, trimmed
• 6 small red new potatoes (about 1/2 lb.), cut into wedges
• 1 tbsp. chopped fresh rosemary
• 1⁄4 lb. mushrooms, such as oyster or shiitake, stemmed and torn
• 1 tbsp. unsalted butter
• 1 tsp. finely chopped fresh thyme
• 9 cups loosely packed mixed greens, such as frisée and arugula (about 5 oz.)
• 1⁄2 cup loosely packed mixed flat-leaf parsley and mint leaves
• 2 tbsp. chopped toasted pistachios