Instant Pot French Onion Soup

Name: Instant Pot French Onion Soup
Ingredients: 1/4 cup unsalted butter or vegetable oil; 2 1/2 to 3 pounds yellow or red onions , halved, peeled, and sliced 1/4-inch thick; 1 tablespoon brown sugar; 1 1/2 teaspoons kosher salt; 1/4 cup dry sherry or vermouth; 4 cups low sodium beef broth; 1 teaspoon Worcestershire sauce; 1/4 teaspoon ground black pepper; 1/4 teaspoon dried thyme leaves or 1 teaspoon fresh thyme leaves; 1 bay leaf; 6 (1-inch thick) slices of a baguette; 1 1/4 teaspoons Dijon mustard; 1 cup grated Gruyere cheese; Freshly ground black pepper