boneless, skinless chicken thighs • Kosher salt and freshly ground black pepper • garlic cloves, finely grated or minced • fresh thyme leaves, plus 3 thyme sprigs • extra-virgin olive oil, plus more for serving • cremini mushrooms, cut into 1-inch chunks • red onion, cut into ½-inch-thick wedges • minced fresh tarragon (or marjoram or sage), plus more for garnish if you like • dry sherry or dry vermouth (or ½ tablespoon lime juice and ½ tablespoon orange juice) • unsalted butter, melted • Flaky salt, for serving