Steak Sandwich with Peppercorn Sauce - Traeger Grills

Ingredients

• 1/3 Cup mayonnaise
• 1 Teaspoon fresh lemon juice
• 1 Teaspoon finely chopped fresh basil
• 1 Teaspoon finely chopped flat-leaf parsley
• Pinch cayenne
• kosher salt and freshly ground black pepper
• 2 Tablespoons unsalted butter
• 1 Large Spanish or yellow onion, halved through the root and thinly sliced lengthwise with the grain
• kosher salt
• 2 Cloves garlic, minced
• 4 Tablespoons unsalted butter
• 1 Tablespoon coarsely ground black peppercorn
• 1 shallot, minced
• 1 Ounce brandy or cognac
• 1/2 Cup beef stock
• 1/4 Cup heavy cream
• Kosher salt, to taste
• 1 Tablespoon olive oil
• 1 Tablespoon unsalted butter
• 8 Ounces cremini mushrooms, sliced
• 1 Tablespoon fresh thyme leaves
• 1 Pound strip steak, boneless rib-eye, or flat iron steak
• 1 Large baguette, split lengthwise
• olive oil
• 2 Cups arugula