Step 1

Set the cooking time: Select the high sauté setting on your pressure cooker. If it’s an Instant Pot or similar model, the timer should default to a cooking time of 30 minutes. You will use every minute of this to sauté the onions.

Step 2

Add the fat and caramelize the onions: Add the butter or oil to the pressure cooker. Once the butter has melted or the oil shimmers, about 2 minutes, add the onions, sugar, and salt and stir with a wooden spoon, separating the onion layers and coating the slices with the fat.Cover the pot with a tempered glass lid or other 9-inch pot lid and let the onions cook, without stirring, for 10 minutes.

Step 3

Uncover the pot and continue cooking the onions: Uncover the pot. At this point the onions should be softened, but not brown, and there should be some liquid at the bottom of the pot.While wearing an oven mitt, hold the rim of the inner pot in place, and stir the onions vigorously. Be careful not to splash hot liquid on yourself.Continue to stir the onions vigorously every 2 minutes, making sure to nudge loose any fond or brown bits from the bottom of the pot as it develops with your spoon. Do this until the sauté cooking program has ended after 30 minutes and the pot turns off automatically.At this point, the onions should be caramelized and golden brown.

Step 4

Deglaze the pot and cook the soup under pressure: While the pot is off but still hot and the onions are sizzling, add the sherry. Use a wooden spoon to thoroughly scrape any remaining fond from the pot bottom.Stir in the broth, Worcestershire, thyme, bay leaf, and pepper. Secure the pressure cooker lid in its sealing position, then select the manual setting, and set the time for 5 minutes at high pressure.(The pot will take about 10 minutes to come up to pressure before the cooking program begins.)

Step 5

Release the pressure and prepare to serve: When the cooking program ends, let the pressure release naturally for at least 15 minutes, then move the pressure release valve to "venting" to release any remaining steam.At this point, the soup will stay hot on the “keep warm” setting for up to 10 hours, until you are ready to serve it.When you are ready to serve the soup, discard the bay leaf. Taste the soup for seasoning, adding salt and/or pepper if needed.

Step 6

Make the toasts and finish the soup: Preheat the broiler on your oven and line a baking sheet with aluminum foil. Place broiler-safe serving bowls on the baking sheet, then ladle the soup into the bowls, filling them to about 1/2-inch from the top.Spread the baguette slices very sparingly with Dijon mustard (1/4 teaspoon per slice), then place the baguette slices on top of the bowls of soup.Heap a pile of the Gruyere cheese onto each baguette slice. Grind one or two grinds of black pepper on top of each slice. Broil for 3-5 minutes, or until the cheese is bubbly and beginning to brown.