Bistro Burger with Blue Cheese, Caramelized Onions and Cornichon Aioli | Williams Sonoma

Ingredients

• 5 Tbs. (2 1/2 oz./75 g) salted butter, divided
• 2 Tbs. olive oil
• 1 yellow onion, thinly sliced
• 1/4 cup (2 fl. oz./60 ml) water
• Kosher salt
• 6 garlic cloves, thinly sliced
• 1 Tbs. fresh thyme leaves, minced
• 2 Tbs. balsamic vinegar
• 1 garlic clove
• 1 cup (5 1/4 oz./150 g) cornichons, drained
• 1 whole egg plus 2 egg yolks
• 1 cup (8 fl. oz./250 ml) neutral oil or mild olive oil
• 1 lb. (500 g) ground beef, at room temperature
• Kosher salt and freshly ground pepper
• Neutral oil as needed
• 1/2 cup (2 1/2 oz./75 g) crumbled or thinly sliced French blue cheese
• <br> 2 Tbs. salted butter
• 4 brioche buns, split
• Handful of arugula for serving