Onion Soup | George Washington's Mount Vernon

Ingredients

• 1/2 cup (1 stick) unsalted butter
• 2 1/2 pounds onions, peeled and coarsely chopped (7 to 8 cups)
• 1/3 cup all-purpose flour
• 4 cups water
• 2 cups Basic Beef Stock
• 1 teaspoon salt
• 1 slice bread, toasted and diced
• 2 large egg yolks, lightly beaten
• 1 tablespoon balsamic vinegar
• Ground black pepper