Veal Mushroom Burger with Balsamic Onions - Catelli Brothers

Ingredients

• 2 tablespoons olive oil, divided
• 2 onions (5 oz. each), cut lengthwise in half then crosswise into ¼” thick slices
• 2 tablespoons balsamic vinegar
• 1/4 teaspoon plus 1/8 teaspoon salt, divided
• 8 ounces mixed mushrooms, trimmed and finely chopped
• 2 cloves garlic, minced
• 2 teaspoons chopped fresh thyme
• 1 teaspoon grated lemon zest
• 1 pound ground veal
• 4 egg twist rolls, split
• 4 Boston lettuce leaves
• 4 slices thinly sliced provolone cheese